Checker Plus for Gmail skin, based on the Darcula theme for JetBrains IDEs, skin based on "Dark Red by Occam"

CheckerPlusDarcula.css

/*background color*/
.mail,
body.popup app-toolbar:not(.accountHeader),
#betaCalendar .fc-month-view .timedEvent .fc-content:hover .fc-title, 
app-header,
.fc-unthemed .fc-list-item:hover td,
body:not(.background-skin) #inboxSection,
body:not(.background-skin) #inboxSection app-header-layout #main-header-toolbar,
.label {
    background-color: var(--primary-background-color) !important;
}

.fc-body {
    background-color: var(--main-background-color) !important;
}

/*open email*/
.page {
	background-color: var(--main-background-color);
	color: var(--secondary-text-color);
}

#openEmailHeader {
	color: var(--primary-text-color);
}

#replyArea, replyArea:hover {
	background-color: var(--primary-background-color) !important;
	border: none;
	color: var(--primary-text-color);
}

.messageBody {
	filter: invert(90%);
}

paper-item:hover,
paper-icon-item:hover,
textarea,
.ui-datepicker,
#skinsDialog .addButton,
#betaCalendar .fc-basic-view .fc-week-number,
#accountAvatars {
    background-color: var(--secondary-background-color) !important;
}
.accountHeader {
    background-color: var(--main-background-color) !important;
}
#accountAvatars {
    height: 100vh;
    margin-top: 0;
    padding-top: 2vh;
}
.fc-unthemed .fc-list-heading td {
    background-color: #444;
}
app-drawer-layout,
#mainContent {
    background-color: var(--main-background-color) !important;
}
#betaCalendar.highlightWeekends .fc-sat,
#betaCalendar.highlightWeekends .fc-sun{
    background-color: #444a;
}
.fc-unthemed .fc-list-empty,
.fc-day-top {
    background-color: #0000 !important;
}
body {
    background-color: #000;
}
/*scroll bar*/
::-webkit-scrollbar,
::-webkit-scrollbar-corner {
    background-color: var(--secondary-background-color);
}
::-webkit-resizer {
    background-color: var(--primary-background-color);
}
::-webkit-scrollbar-track {
    box-shadow: inset 0 0 10px var(--line-color);
    border-radius: 10px;
}
::-webkit-scrollbar-thumb {
    background-color: var(--accent-color);
    border-radius: 10px;
}
::-webkit-scrollbar-thumb:hover {
    background-color: var(--primary-color);
}
/*text color, boldness, and opacity*/
table {
    color: var(--secondary-text-color);
}

#titleLabel,
.accountTitle,
.unreadCount,
.unread,
.labelName {
    color: var(--primary-text-color) !important;
}
#betaCalendar .fc-basic-view .fc-week-number,
#betaCalendar .fc-basic-view .fc-day-number,
#calendarTitle,
.ui-datepicker-title,
body.popup app-toolbar,
.ui-datepicker,
thead,
.fc-unthemed .fc-list-heading td,
.fc-unthemed .fc-list-empty {
    color: var(--primary-text-color) !important;
    font-weight: 500;
}

.dayOfYear,
#betaCalendar .fc-month-view .timedEvent .fc-time,
#betaCalendar .fc-month-view .timedEvent .fc-title,
#betaCalendar .fc-custom-view .timedEvent .fc-time,
#betaCalendar .fc-custom-view .timedEvent .fc-title,
paper-item:hover,
paper-icon-item:hover,
textarea,
#mainContent,
app-header,
#betaCalendar.highlightPastDays .fc-event.pastEvent .fc-content .fc-time,
#betaCalendar.highlightPastDays .fc-event.pastEvent .fc-content .fc-title,
#betaCalendar .fc-list-item {
    color: var(--secondary-text-color) !important;
    font-weight: 500;
}
html:not(.searchInputVisible) #inboxSection app-header-layout app-toolbar paper-icon-button:not(#menu),
.snippet {
    color: var(--secondary-text-color) !important;
}
#betaCalendar.highlightPastDays .fc-past,
#betaCalendar.highlightPastDays .pastEvent .eventColorIndicator,
#betaCalendar.highlightPastDays .pastEvent .fc-time,
#betaCalendar.highlightPastDays .pastEvent .fc-content .fc-title {
    opacity: 0.5 !important;
}
/*sidebar calendar*/
.ui-state-default {
    color: var(--secondary-text-color) !important;
}
.ui-state-default:hover {
    color: var(--secondary-text-color) !important;
}
/*today highlight*/
.ui-state-highlight,
.ui-widget-content .ui-state-highlight,
.ui-widget-header .ui-state-highlight,
#betaCalendar .fc-today .fc-day-number {
    background-color: var(--accent-color) !important;
}
.ui-state-highlight:hover,
.ui-widget-content .ui-state-highlight:hover,
.ui-widget-header .ui-state-highlight:hover {
    background-color: var(--primary-color) !important;
}
#betaCalendar th.fc-today,
.agendaDay.today .agendaDateWrapper {
    color: var(--primary-color) !important;
}
/*lines*/
textarea,
textarea:focus,
.mail,
.fc-unthemed .fc-popover,
.fc-unthemed .fc-row,
.fc-unthemed hr,
.fc-unthemed tbody,
.fc-unthemed td,
.fc-unthemed th,
.fc-unthemed thead,
.fc-unthemed table,
.fc-view {
    border-color: var(--line-color) !important;
    outline-color: var(--line-color);
}
.separator,
.fc-unthemed .fc-divider {
    background-color: var(--line-color);
}
/*colors*/
paper-icon-button,
paper-dialog .buttons paper-icon-button {
    color: var(--primary-color);
    --iron-icon_-_opacity: 0.7;
}
.labelName {
    background-color: var(--accent-color);
}
/*OK button*/
paper-button[raised].colored {
    background-color: var(--accent-color) !important;
    color: var(--primary-text-color);
    font-weight: 700;
}
/*links*/
a,
a:visited {
    color: var(--primary-color);
}
/* search bar */
#searchInputWrapper #searchInput {
    color: var(--secondary-text-color) !important;
}

/*user variables*/
* {
    --primary-background-color: #3C3F41;
    --secondary-background-color: #313335;
    --primary-text-color: #A9B7C6;
    --secondary-text-color: #ADADAD;
    --primary-color: #A4A3A3;
    --accent-color: #4B4F51;
    --line-color: #555555;
    
    --main-background-color: #2B2B2B;
    --account-header-background-color: #313335;
    --background-color: #00000000;
    --today-color-with-transparency: #3C3F41;
    --today-color-full-transparency: #4440;
}

Dark Brave color theme:

* {
  --primary-text-color: #ededed;
  --primary-background-color: #222222;
  --secondary-text-color: #909192;
  --secondary-background-color: #282828;
  --disabled-text-color: #6f6f6f;
  --highlight-background-color: #393939;
  --compose-color: #444444;
  --hover-message-color: #444444;
  --error-color: #d04a48;
  --primary-color: #f15220;
  --accent-color: #f95234;
  --icons: #ffffff;
}