Dark Theme for Proton Mail

proton-mail.css

/* ProtonMail */
/* CLASSIC THEME */

body { font-size: 14px }

body,
.pm_opensans {
    font-family: 'Lucida Grande', Verdana, Arial, sans-serif
}

/* UI Elements */

.pm_buttons,
.pm_button,
.pm_toggle span.knob {
    border-color: #999;
}

.pm_button {
    background-image: -webkit-linear-gradient(#fff, #ddd);
    background-image: linear-gradient(#fff, #ddd);
    background: -moz-linear-gradient(#fff, #ddd)
}

.pm_button:active {
    box-shadow: inset 0 0 2rem rgba(0,0,0,.1) ;
}

.pm_button.primary {
    background: #7575a3;
    background-image: -webkit-linear-gradient(#7575a3, #669);
    background-image: linear-gradient(#7575a3, #669);
}

.pm_button.success {
    background: #93bf73;
    background-image: -webkit-linear-gradient(#93bf73, #aecf96);
    background-image: linear-gradient(#93bf73, #aecf96)
}

.pm_button.error {
    background: #c26164;
    background-image: -webkit-linear-gradient(#c26164, #ad4346);
    background-image: linear-gradient(#c26164, #ad4346);
    color: #fff ;
    border-color: #ad4346;
}

.pm_button.primary,
.pm_button.success {
    text-shadow: none;
    color: #fff;
    border-color: transparent
}

.pm_button.link,
.pm_button.primary,
.pm_button.success {
    border-color: transparent
}

.pm_button.link {
    background-color: transparent;
    color: #8286c5;
    text-decoration: underline;
    box-shadow: none;
    background-image: none;
}

.pm_button,
.pm_buttons :not(.pm_dropdown) a,
.pm_buttons :not(.pm_dropdown) button {
    box-shadow: 0 1px 1px rgba(0, 0, 0, .1), inset 0 1px 0 rgba(255, 255, 255, .1);
}

.pm_buttons a {
    background-image: -webkit-linear-gradient(#fff, #ddd);
    background-image: linear-gradient(#fff, #ddd);
    background: -moz-linear-gradient(#fff, #ddd)
}

.pm_buttons a.active,
.pm_buttons button.active,
.pm_buttons a:active,
.pm_buttons button:active,
.navigation-link:active,
.navigation-link.active {
    box-shadow: 0 1px 1px rgba(0, 0, 0, .1), inset 0 1px 0 rgba(255, 255, 255, .1), inset 0 2px 2px 0 rgba(0,0,0,.2), inset 0 0 2rem rgba(0,0,0,.1)
}

.pm_buttons .pm_dropdown a {
    background: none;
}

.pm_table th { color: #555 }

.pm_toggle { box-shadow: 0 0 0 1px #999 }

.pm_toggle.off .on,
.pm_toggle.off .off,
.pm_toggle.on .on,
.pm_toggle.on .off {
    opacity: 1;
}


/* Layout */

.sidebarApp-container,
.mobileMode .sidebarApp-container,
.headerDesktop-logo,
.composerHeader-container { background: #333 }

.search-form-fieldset,
.searchForm-action-button-toggle:hover,
.searchForm-action-button-advanced:hover { background-color: rgba(0,0,0,0.2) }

.headerDesktop-container { background: #555 }

.composer-container {
    box-shadow: none !important;
    border: 1px solid #333;
}

/* Mobile */

#pm_header-mobile { background: #555 }


/* Conversations */

.conversation.read,
.conversation.read .pm_labels label,
#conversation-list-rows .conversation.read .subject .pm_labels,
#conversation-list-rows .conversation.read .subject .pm_labels label,
#conversation-list-columns .conversation.read .subject .pm_labels,
#conversation-list-columns .conversation.read .subject .pm_labels label {
    background: #ddd;
}

#conversation-list-rows .conversation.active,
#conversation-list-columns .conversation.active {
    background: #555;
}

.conversation.read .pm_labels label { box-shadow: 0.25rem 0 0 0.25rem #ddd }
.conversation.active .pm_labels label {
    box-shadow: 0.25rem 0 0 0.25rem #555;
    background: #555;
}

#conversation-list-rows .conversation { border-color: #999 }


/*
 * ----------------------------------------
 * Dark Theme Changes
 * ----------------------------------------
 */

.sidebarApp-container,
.mobileMode .sidebarApp-container,
.headerDesktop-logo,
.composerHeader-container,
.headerDesktop-container {
  background: #17172c;
}

#pm_main {
  color: #fff;
  background: #252535;
}

body #conversation-list-rows .conversation {
  background: #4f527b;
}

body #conversation-list-rows .conversation.read {
  background: #252535;
}

body #conversation-list-rows .conversation .subject h4 {
  color: #fff;
}

.conversation.read,
.conversation.read .pm_labels label,
#conversation-list-rows .conversation.read .subject .pm_labels,
#conversation-list-rows .conversation.read .subject .pm_labels label,
#conversation-list-columns .conversation.read .subject .pm_labels,
#conversation-list-columns .conversation.read .subject .pm_labels label {
  background: #252535;
}

#conversation-view .frame {
  background: #fff;
  color: #555;
}

#conversation-view .message .summary,
#conversation-view .message .summary .labels,
#conversation-view .message .summary .status,
#conversation-view .message .summary .time,
#conversation-view .message .details {
  background: inherit;
}

#conversation-view header h1,
#conversation-view header h1 .starButton a {
  color: #fff;
}

#conversation-view .message .summary .time::after {
  background: none;
}

.customRadio-input:checked + .customRadio-mask {
  border-color: #252535;
}

.pm_toggle span.knob {
  background: #252535;
}

a,
.alert.alert-danger a, .alert.alert-info a, .alert.alert-success a, .alert.alert-warning a {
  color: #7575a3;
}

.alert.alert-info {
  background: #4b4c57;
  color: #fff;
}

.pm_buttons,
.pm_button,
.pm_toggle span.knob {
    border-color: #252535;
}

.pm_toggle { box-shadow: 0 0 0 1px #252535 }

.pm_table table.plain th,
.pm_table table td {
  color: #fff;
}

#pm_settings .donate .well {
  box-shadow: none;
}

.pm_form select,
.pm_form .pm_select select {
  color: #fff;
  background: #252535;
  border: 1px solid #fff;
}

.pm_form .pm_select .fa {
  z-index: 3;
}

#plans .plans .plan.plus-plan footer,
#plans .plans .plan.plus-plan header {
  background: #252535;
}

#plans .plans .plan footer .cycle label,
#plans .plans .plan footer .cycle {
  color: #fff;
}

.pm_sort ul li.highlight {
  background-color: #252535;
}

.pm_table table tr.hasTable table {
  background-color: #252535;
}

code {
  color: #fff;
}

.searchForm-advanced-wrapper {
  background: #252535;
}

.searchForm-advanced-container label,
.pm_form .pm_select .fa {
  color: #fff;
}

.settingsDashboard-overview-section-table-container .overviewSection-container .overviewSection-tbody-billing,
.settingsDashboard-plans,
.settingsDashboard-plans::after, .settingsDashboard-plans::before {
  background: initial;
}

Good theme thanks !