Add styles to the WordPress Customizer that mimics the WordPress admin.

README.md

Default Admin Styles for WordPress Customizer

WordPress Snippet - Stylesheet

customizer.css

.wp-full-overlay-sidebar {
	background: #23282d;
	border: none;
}

.wp-full-overlay-sidebar ::-moz-selection {
	background: #0073aa;
	color: #eee;
	text-shadow: none;
}

.wp-full-overlay-sidebar ::selection {
	background: #0073aa;
	color: #eee;
	text-shadow: none;
}

.wp-full-overlay-sidebar {
	-webkit-tap-highlight-color: #0073aa;
}

#customize-controls .customize-info {
	border: none;
}

#customize-header-actions {
	border: none;
}

.wp-full-overlay-sidebar .wp-full-overlay-header {
    background-color: #23282d;
}

#accordion-section-themes + .control-section,
#customize-controls .customize-controls-close,
#customize-controls .customize-controls-preview-toggle,
#customize-controls .customize-controls-close:focus,
#customize-controls .customize-controls-preview-toggle:focus,
#customize-theme-controls .control-panel-content .control-section:nth-child(2),
#customize-theme-controls .control-panel-nav_menus .control-section:nth-child(3),
#customize-theme-controls .control-section.open {
	background: #23282d;
	border: none;
	color: #eee;
}

#customize-controls .customize-controls-close:hover,
#customize-controls .customize-controls-preview-toggle:hover,
#customize-controls .customize-info .accordion-section-title,
#customize-controls .customize-section-title,
#customize-controls .panel-meta.customize-info .accordion-section-title:hover,
#customize-controls .customize-panel-back,
#customize-controls .customize-section-back {
	background: #0073aa;
	color: #eee;
	border: none;
}

#customize-controls .customize-info .customize-help-toggle,
#customize-controls .customize-info .customize-help-toggle:focus,
#customize-controls .customize-info .customize-help-toggle:hover,
#customize-controls .customize-info.open .customize-help-toggle,
.customize-screen-options-toggle,
.customize-screen-options-toggle:hover {
    color: #eee;
}

@media screen and (max-width: 640px) {
	#available-menu-items .customize-section-title h3,
	#available-widgets .customize-section-title h3 {
		color: #eee;
	}
}

#customize-controls .customize-info .customize-panel-description,
#customize-controls .no-widget-areas-rendered-notice {
    color: #eee;
    background: #32373c;
    border: none;
}

#customize-controls .customize-panel-back:hover,
#customize-controls .customize-section-back:hover {
	background: #32373c;
}

#customize-controls .control-section .customize-section-title h3,
#customize-controls .customize-info .panel-title {
	font-weight: 400;
}

#customize-theme-controls #accordion-section-themes .accordion-section-title,
#customize-theme-controls #accordion-section-themes .accordion-section-title:after {
	color: #eee;
	background-color: #32373c !important;
	border: none;
}

#customize-theme-controls .accordion-section-title,
#customize-theme-controls .accordion-section-title:after {
	font-weight: 400;
	color: #eee;
	background-color: #23282d;
	border: none;
	-webkit-transition: all .15s;
	transition: all .15s;
}

.customize-section-title,
.customize-section-back {
    border: none;
    background: #0073aa;
    color: #eee;
}

.customize-section-back:hover {
	background: #32373c;
	color: #eee;
}

#customize-controls .accordion-section-title:before,
#customize-theme-controls .accordion-section-title:after {
	color: #a0a5aa;
}

#customize-controls .control-section .accordion-section-title:focus,
#customize-controls .control-section .accordion-section-title:hover,
#customize-controls .control-section.open .accordion-section-title,
#customize-controls .control-section:hover > .accordion-section-title,
#customize-controls .control-section .accordion-section-title:focus:after,
#customize-controls .control-section .accordion-section-title:hover:after,
#customize-controls .control-section.open .accordion-section-title:after,
#customize-controls .control-section:hover > .accordion-section-title:after,
#customize-controls .accordion-section-title:hover:before {
	color: #00b9eb;
	background: #191e23;
}

#customize-controls .control-section .customize-section-title h3,
#customize-controls .control-section h3.customize-section-title,
#customize-controls .customize-info .panel-title,
#customize-theme-controls .accordion-section-content {
	color: #eee;
}

.customize-control.customize-control-nav_menu_name {
	background: #23282d;
}

.customize-control {
	background: #32373c;
	border-bottom: solid 2px #23282d;
	padding: 0.5em 0.7em;
}

.menu-settings .customize-control {
	border: none;
}

.customize-section-description-container + #customize-control-custom_css:last-child {
	padding: 0;
}

.customize-control.customize-control-nav_menu_item,
.wp-customizer .menu-item-bar .menu-item-handle {
	background: #32373c;
	border: none;
}

.customize-control.customize-control-nav_menu_item {
	border-bottom: solid 2px #23282d;
}

.menu-item-handle,
.menu-item-handle:hover,
.widget .widget-top,
.widget .widget-top:hover,
.item-type,
.menu-item-handle:hover .item-type,
.accordion-section-title:after,
.handlediv,
.handlediv:hover,
.item-edit,
.item-edit:hover,
.sidebar-name-arrow,
.accordion-section-title:hover:after,
.handlediv:focus,
.handlediv:hover,
.item-edit:focus,
.item-edit:hover,
.sidebar-name:hover .sidebar-name-arrow,
.widget-action:focus,
.widget-top:hover .widget-action,
.wp-customizer .toggle-indicator:after {
	color: #eee;
}

.wp-customizer .menu-item-settings {
	color: #eee;
	background: #32373c;
	border: none;
}

#customize-controls .description {
	color: #eee;
}

.wp-core-ui .button,
.wp-core-ui .button-secondary {
	background: #0085ba;
	border-color: #0073aa #006799 #006799;
	-webkit-box-shadow: 0 1px 0 #006799;
	box-shadow: 0 1px 0 #006799;
	color: #fff;
	text-decoration: none;
	text-shadow: 0 -1px 1px #006799,1px 0 1px #006799,0 1px 1px #006799,-1px 0 1px #006799;
}

.wp-core-ui .button:focus,
.wp-core-ui .button:hover,
.wp-core-ui .button-secondary:focus,
.wp-core-ui .button-secondary:hover,
.add-menu-toggle.open,
.add-menu-toggle.open:hover,
.adding-menu-items .add-new-menu-item,
.adding-menu-items .add-new-menu-item:hover,
body.adding-widget .add-new-widget,
body.adding-widget .add-new-widget:hover {
	background: #008ec2;
	border-color: #006799;
	color: #fff;
}

.wp-core-ui .button-link.menu-delete,
.wp-core-ui .button-link.item-delete,
.wp-customizer .menu-item .submitbox .submitdelete {
	background: #c00;
	border-radius: 3px;
	color: #fff;
	padding: 3px 10px;
	text-decoration: none;
}

.wp-core-ui .button-link.menu-delete:hover,
.wp-core-ui .button-link.item-delete:hover,
.wp-customizer .menu-item .submitbox .submitdelete:hover {
	background: #f00;
}

.wp-core-ui .button-link.reorder-toggle,
.link-to-original,
.original-link {
	color: #eee;
}

.wp-core-ui .button-link.reorder-toggle:hover,
.original-link {
	color: #008ec2;
}

.collapse-sidebar.button-secondary {
	text-shadow: none;
}

#available-menu-items .item-add:focus:before,
#available-menu-items-search .clear-results:focus,
#customize-controls .customize-info .customize-help-toggle:focus:before,
.customize-screen-options-toggle:focus:before,
.menu-delete:focus,
.menu-item-bar .item-delete:focus:before,
.wp-customizer .menu-item .submitbox .submitdelete:focus,
.wp-customizer button:focus .toggle-indicator:after {
	-webkit-box-shadow: none;
	box-shadow: none;
}

input[type="text"],
input[type="search"],
input[type="radio"],
input[type="tel"],
input[type="time"],
input[type="url"],
input[type="week"],
input[type="password"],
input[type="checkbox"],
input[type="color"],
input[type="date"],
input[type="datetime"],
input[type="datetime-local"],
input[type="email"],
input[type="month"],
input[type="number"],
select,
textarea {
	border: 1px solid #23282d;
	-webkit-box-shadow: none;
	box-shadow: none;
	background-color: #23282d;
	color: #eee;
}

#customize-control-custom_css textarea {
	background: #23282d;
	color: #eee;
}

.wp-color-result {
	background: #eee;
	border: none;
	-webkit-box-shadow: none;
	box-shadow: none;
}

.wp-color-result:hover {
	border-color: #23282d;
}

.wp-color-result::after {
    background: #23282d;
    border: none;
    -webkit-border-radius: 0px;
    border-radius: 0px;
    color: #eee;
}

#available-widgets-list {
	background: #23282d;
	border: none;
}

#available-menu-items .item-tpl,
#available-widgets .widget-tpl {
	transition: none !important;
}

#available-widgets .widget-tpl.selected,
#available-menu-items,
#available-widgets,
#available-widgets-filter {
	background: #23282d;
	border: none;
}

#available-menu-items .item-tpl,
#available-widgets .widget-tpl,
#available-widgets .widget-tpl.selected {
    border-left-color: #32373c;
    border-bottom: 1px solid #23282d;
}

#available-menu-items .item-tpl:hover,
#available-menu-items .item-tpl:focus,
#available-widgets .widget-tpl:hover,
#available-widgets .widget-tpl:focus {
    background: #0073aa;
	color: #eee;
    border: none;
}

#available-widgets .widget .widget-description,
.widget-title h3,
#available-widgets .widget-title:before,
.customize-control .widget-action,
.widget-top a.widget-action,
.widget-top a.widget-action:hover,
.widget-control-remove,
.widget-control-close {
	color: #eee;
}

#available-menu-items .item-tpl,
#available-widgets .widget-tpl {
	background-color: #32373c;
}

.widget-control-remove:hover {
	color: #f00;
}

.widget-control-close:hover {
	color: #008ec2;
}

.widget-top a.widget-action:focus:after {
	color: #eee;
	-webkit-box-shadow: none;
	box-shadow: none;
}

.widget-inside,
.customize-control-widget_form .widget-top {
	background: #32373c;
	border: none;
}

.widget-action:focus {
	border: none;
}

.wp-core-ui .button, .wp-core-ui .button-secondary {
    color: #fff;
    text-shadow: none;
}

.expanded .wp-full-overlay-footer,
.wp-full-overlay-footer .devices,
.wp-core-ui .wp-full-overlay .collapse-sidebar {
	background: #0073aa;
	color: #fff;
	border: none;
	-webkit-box-shadow: none;
	box-shadow: none;
}

.wp-full-overlay-footer .devices button:before,
.wp-full-overlay-footer .devices button.active:before {
	color: #fff;
}

.wp-full-overlay-footer .devices button.active {
	border-bottom-color: #fff;
}

.wp-full-overlay-footer .devices button:hover,
.wp-full-overlay-footer .devices button.active:hover,
.wp-full-overlay-footer .devices button.active:focus {
	border-bottom-color: #0073aa;
}

.wp-full-overlay-footer .devices button:hover:before,
.wp-full-overlay-footer .devices button:focus:before,
.wp-full-overlay-footer .devices button.active:hover:before,
.wp-full-overlay-footer .devices button.active:focus:before {
	color: #0073aa;
}

.wp-full-overlay .collapse-sidebar-arrow:before {
	background: #0073aa;
	color: #fff;
}

.wp-full-overlay .collapse-sidebar-arrow:hover:before {
	background: #fff;
	color: #0073aa;
}

.wp-core-ui .wp-full-overlay .collapse-sidebar:focus,
.wp-core-ui .wp-full-overlay .collapse-sidebar:hover {
	color: #fff;
}

#customize-theme-controls .accordion-section-content,
#customize-controls .customize-section-title {
	padding: 0;
}

.customize-control {
    box-sizing: border-box;
}

@media screen and (max-width: 640px) {
	body.adding-menu-items div#available-menu-items,
	body.adding-widget div#available-widgets {
		top: 45px;
	}
}

enqueue-customizer-styles.php

<?php

// Place this in your theme's functions.php file
function my_customizer_script_styles() {
  wp_enqueue_style( 'customizer',  get_stylesheet_directory_uri() . '/customizer.css' );
}
add_action( 'customize_controls_enqueue_scripts', 'my_customizer_script_styles' );

?>