element-web/res/themes/light-high-contrast/css/_light-high-contrast.scss

//// Reference: https://www.figma.com/file/RnLKnv09glhxGIZtn8zfmh/UI-Themes-%26-Accessibility?node-id=321%3A65847
$accent: #268075;
$alert: #D62C25;
$links: #0A6ECA;
$primary-content: #17191C;
$secondary-content: #5E6266;
$tertiary-content: $secondary-content;
$quaternary-content: $secondary-content;
$quinary-content: $secondary-content;
$roomlist-button-bg-color: rgba(141, 151, 165, 0.2);

$username-variant1-color: #0A6ECA;
$username-variant2-color: #AC3BA8;
$username-variant3-color: #078662;
$username-variant4-color: #CC1449;
$username-variant5-color: #BE4C00;
$username-variant6-color: #1C7274;
$username-variant7-color: #5C56F5;
$username-variant8-color: #3E810A;

$accent-color: $accent;
$accent-color-50pct: rgba($accent-color, 0.5);
$accent-color-alt: $links;
$input-border-color: $secondary-content;
$input-darker-bg-color: $quinary-content;
$input-darker-fg-color: $secondary-content;
$input-focused-border-color: $accent-color;
$resend-button-divider-color: $input-darker-bg-color;
$icon-button-color: $quaternary-content;
$theme-button-bg-color: $quinary-content;
$presence-offline: $quinary-content;
$pinned-color: $tertiary-content;
$button-secondary-bg-color: $accent-fg-color;
$message-action-bar-fg-color: $primary-content;
$reaction-row-button-selected-border-color: $accent-color;
$voice-record-stop-border-color: $quinary-content;
$voice-record-icon-color: $tertiary-content;
$appearance-tab-border-color: $input-darker-bg-color;
$eventbubble-reply-color: $quaternary-content;
$notice-primary-color: $alert;
$warning-color: $notice-primary-color; // red
$roomtopic-color: $secondary-content;

@define-mixin mx_DialogButton_danger {
    background-color: $accent-color;
}

@define-mixin mx_DialogButton_secondary {
    // flip colours for the secondary ones
    font-weight: 600;
    border: 1px solid $accent-color !important;
    color: $accent-color;
    background-color: $button-secondary-bg-color;
}

@define-mixin mx_Dialog_link {
    color: $accent-color;
    text-decoration: none;
}

.mx_AccessibleButton {
    margin-left: 4px;
}

.mx_AccessibleButton:focus {
    outline: 2px solid $accent-color;
    outline-offset: 2px;
}

.mx_BasicMessageComposer .mx_BasicMessageComposer_inputEmpty > :first-child::before {
    color: $secondary-content;
    opacity: 1 !important;
}

.mx_TextualEvent {
    color: $secondary-content;
    opacity: 1 !important;
}

.mx_Dialog, .mx_MatrixChat_wrapper {
    :not(.mx_textinput):not(.mx_Field):not(.mx_no_textinput) > input[type=text]::placeholder,
    :not(.mx_textinput):not(.mx_Field):not(.mx_no_textinput) > input[type=search]::placeholder,
    .mx_textinput input::placeholder {
        color: $input-darker-fg-color !important;
    }
}

.mx_UserMenu_contextMenu .mx_UserMenu_contextMenu_header .mx_UserMenu_contextMenu_themeButton {
    background-color: $roomlist-button-bg-color !important;
}

.mx_FontScalingPanel_fontSlider {
    background-color: $roomlist-button-bg-color !important;
}

.mx_ThemeChoicePanel > .mx_ThemeSelectors > .mx_StyledRadioButton input[type="radio"]:disabled + div {
    border-color: $primary-content;
}

.mx_ThemeChoicePanel > .mx_ThemeSelectors > .mx_StyledRadioButton.mx_StyledRadioButton_disabled {
    color: $primary-content;
}