My attempt in theming OpenWrt's luci-bootstrap-dark theme.

user.css

/* Either append this to the end of the cascade.css file or inject it using Stylus or any other styling extensions. */

body {
    background: hsl(208, 23%, 13%);
   --background-color-high: hsl(208, 23%, 13%); 
    --background-color-low: hsl(208, 23%, 13%); 
    --border-color-high: hsl(208, 23%, 23%); 
    --border-color-medium: hsl(208, 23%, 17%); 
    --border-color-low: hsl(208, 23%, 10%); 
    --on-primary-color: white;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
}
body, input, button {
     font-family: "Segoe UI Variable Display", "Google Sans", "Product Sans", -apple-system, BlinkMacSystemFont, "Segoe UI", "Ubuntu", sans-serif;
}
.control-group > * {
    margin-top: 4px;
}
h1, h2, h3{
    font-weight: 350;
}
header .brand {
 padding: 0px 20px 1px;
    font-size:20px;
    line-height:40px;
}
 
.label, header [data-indicator] {
    font-size: 11px;
 
    padding: 4px 8px;
}
.actions, .cbi-page-actions {
    border: none;
    padding: 8px 16px;  
}
.cbi-section-node {
    padding: 4px 8px;
}
table, .table, .ifacebox, .cbi-section-node, .create-item-input {
    background: hsl(208, 23%, 15%);
    border-radius: 16px;
}
#channel_graph {
        background: hsl(208, 23%, 15%) !important;
    border-radius: 16px;
    border: none !important;
}
.cbi-dropdown > ul > li input[type="text"] {
    height: 30px;
     border-radius: 6px;
    border: none;
    
}
 .ifacebox  .ifacebox-head {
    border-radius: 16px 16px 0px 0px;
    opacity: 0.9;
}
.cbi-rowstyle-2, .tr.table-titles, .tr.cbi-section-table-titles {
  background: transparent;
}
header, .nav .dropdown-menu {
    border: none;
    backdrop-filter: blur(15px);
    background-image: none;
    box-shadow: none;
    background: hsla(208, 23%, 11%, 0.75);
}
.nav .dropdown-menu {
    background: hsl(208, 23%, 11.5%);
}
.cbi-dropdown[open] > ul.dropdown {
    background: hsl(208, 23%, 23%);
    border-radius:0px 0px 6px 6px;
    box-shadow: none;
     
    border: none;
}
.btn, .cbi-button, .cbi-input-text, textarea, select, #localtime, .cbi-dropdown, .cbi-dropdown:not(.btn):not(.cbi-button), .cbi-page-actions .cbi-button-apply, .cbi-page-actions .cbi-button-apply + .cbi-button-save, .cbi-page-actions .cbi-button-negative + .cbi-button-save, .cbi-dynlist > .item{
 
    border-radius: 6px;
    background: hsl(208, 23%, 23%);
    text-shadow: none;
    padding: 2px 10px;
    box-shadow: none;
    color: white;
    
    border: none;
}
.cbi-dynlist > .item {
    padding: 6px 10px;
}
.cbi-dynlist > .item::after {
    background: none;
    box-shadow: none;
    border-left: solid 1px hsl(208, 23%, 27%);
}
.label.notice, header [data-indicator][data-style="active"] {
    background: hsl(208, 23%, 23%);
}
.dropdown-menu li a:hover {
    background-image: none;
    background: hsl(208, 23%, 23%) !important;
}
.cbi-page-actions .cbi-button-apply {
    height: 30px;
}
.cbi-progressbar {
     background: hsl(208, 23%, 12%);
    border: none;
    border-radius:20px;
}
.cbi-progressbar > div {
     border-radius:20px;
}
.table .td, .table .th {
    border-top: none;
}
.td.cbi-section-actions > * > *, .td.cbi-section-actions > * > form > * {
 margin: 0px;
    border-radius: 0;
    border-right: solid 1px hsl(208, 23%, 13%);
}
.td.cbi-section-actions > * > *:first-child {
     border-radius: 6px 0px 0px 6px
}
.td.cbi-section-actions > * > *:last-child {
     border-radius: 0px 6px 6px 0px;
    border: none;
}
#modal_overlay {
    background: hsla(208, 23%, 2%, 0.6);
}
 .ifacebadge, .ifacebadge.large, .network-status-table .ifacebox-body .ifacebadge {
    background: hsla(208, 23%, 2%, 0.2);
     padding: 8px ;
 
     border: none;
     box-shadow: none;
     border-radius: 6px;
}
.network-status-table {
     width: calc(100% + 16px);
    position: relative;
    left: -8px;
    margin-bottom: 8px;
}
.ifacebox .ifacebox-head.active {
    background: none;
    border: none;
    color: white;
    text-shadow: none;
}
.network-status-table .ifacebox {
    border: none;
    box-shadow: none;
    padding: 8px;
   
    
}
.cbi-tabmenu {
    background: none;
}

.tabs > li:not(.active), .cbi-tabmenu > .cbi-tab-disabled {
    background: hsl(208, 23%, 15%);
    border: none;
    border-radius: 0px;
}
.tabs > li, .cbi-tabmenu > li {
    border: none;
    margin: 0px;
    background: hsl(208, 23%, 23%);
    border-radius: 0px;
    color: white;
    padding: 6px 8px;
    font-size:14px;
    transition: background 200ms ease-in-out;
    height: 32px;
}
.tabs > li:last-child, .cbi-tabmenu > li:last-child {
    border-radius: 0px 6px 6px 0px;
}
.tabs > li:first-child, .cbi-tabmenu > li:first-child {
    border-radius: 6px 0px 0px 6px;
}
.tabs, .cbi-tabmenu {
    justify-content: center;
}
.cbi-section-node .tabs > li:not(.active), .cbi-section-node .cbi-tabmenu > .cbi-tab-disabled{
    background: hsl(208, 23%, 18%);
}
.modal {
    background: hsla(208, 23%, 13%, 0.7); 
    box-shadow: none;
    border-radius: 16px;
    backdrop-filter: blur(20px);
}
input[type="text"] + .cbi-button, input[type="password"] + .cbi-button, select + .cbi-button {
    margin-left: -6px;
    border-radius: 0 6px 6px 0
}
.cbi-section h3 {
    padding-left: 16px;
}
.cbi-modal h4 {
    text-align: center;
}
form .clearfix, .cbi-value {
    margin: 16px 0px;
}
.cbi-page-actions .cbi-button-apply + .cbi-button-save, .cbi-page-actions .cbi-button-negative + .cbi-button-save {
    background:#4b9970;
}

.btn.primary, .cbi-button-action.important, .cbi-page-actions .cbi-button-apply, .cbi-section-actions .cbi-button-edit {
    background: #58809b;
}
.cbi-dropdown[open] > ul.dropdown > li {
    border: none;
    padding: 6px 8px;
    border-bottom: solid 1px hsl(208, 23%, 18%);
}