unfuck firefox proton (dark mode only)

userChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* Turn "pills" back into tabs: */
.tab-background {
  margin-bottom: 0 !important;
  border-radius: var(--tab-border-radius) var(--tab-border-radius) 0 0 !important;
}

.tab-content {
  padding-top: 2px !important;
}

#tabbrowser-arrowscrollbox {
  max-height: calc(var(--tab-min-height) + 4px) !important;
}

/* add rounded joints to the active tab */
.tabbrowser-tab[selected="true"] .tab-background::before,
.tabbrowser-tab[selected="true"] .tab-background::after {
  display: table-caption;
  content: " ";
  width: 5px;
  height: 5px;
  background: transparent;
  border-bottom: 5px solid var(--autocomplete-popup-highlight-background);
  pointer-events: none;
}

.tabbrowser-tab[selected="true"] .tab-background::before {
    border-right: 5px solid var(--autocomplete-popup-highlight-background);
    border-bottom-right-radius: 10px;
    margin-left: -5px;
    margin-top: calc(var(--tab-min-height) - 5px);
}

.tabbrowser-tab[selected="true"] .tab-background::after {
    border-left: 5px solid var(--autocomplete-popup-highlight-background);
    border-bottom-left-radius: 10px;
    margin-top: -10px;
    margin-left: calc(100% - 5px);
}

/* Add a separator between inactive tabs */
#tabbrowser-tabs .tabbrowser-tab {
  padding-inline: 1px 0px !important;
  border-image: linear-gradient(rgba(255, 255, 255, 0), rgba(255, 255, 255, 0.1), rgba(255, 255, 255, 0)) 1 !important;
  border-image-width: 0 0 0 1px !important;
}

#tabbrowser-tabs .tabbrowser-tab:first-of-type,
#tabbrowser-tabs .tabbrowser-tab[first-visible-unpinned-tab="true"],
#tabbrowser-tabs .tabbrowser-tab:hover,
#tabbrowser-tabs .tabbrowser-tab:hover + .tabbrowser-tab,
#tabbrowser-tabs .tabbrowser-tab[selected="true"],
#tabbrowser-tabs .tabbrowser-tab[selected="true"] + .tabbrowser-tab,
#tabbrowser-tabs .tabbrowser-tab[multiselected="true"],
#tabbrowser-tabs .tabbrowser-tab[multiselected="true"] + .tabbrowser-tab {
  border-image-width: 0 !important;
}

/* Make selected tab's color match the urlbar background (dark theme only): */
.tab-background[multiselected="true"],
.tab-background[selected="true"] {
  background-color: var(--autocomplete-popup-highlight-background) !important;
  background-image: none !important;
}

/* Reduce "close tab" button size: */
.tab-close-button {
  padding-inline-start: unset !important;
  width: auto !important;
}

.tab-close-button {
  margin-inline-end: calc(var(--inline-tab-padding) / -2) !important;
  width: 18px !important;
  height: 18px !important;
  padding: 3px !important;
  border-radius: var(--tab-border-radius) !important;
  margin-top: 1px !important;
}