Dark mode for Basecamp 3 with Stylus

stylus_basecamp-dark.css

/* Work in progress.
   A Basecamp 3 dark theme for Stylus
   (https://addons.mozilla.org/en-US/firefox/addon/styl-us/)
   based on https://userstyles.org/styles/177262/basecamp-insomnia */

:root
  {
    --bg-md: #18242b;
    --bg-md-2: #152025;
    --bg-lt: #213038;
    --bg-dk: #0f171b;
    --bg-primary: #10364a;
    --text-md: #949cb7;
    --text-lt: #cbd3f1;
    --text-dk: #555b6f;
    --text-primary: #2285c3;
    --bg-client-show: #ffdb4d;
    --text-client-show: #283c46;
  }

  body
  {
    color: var(--text-md);
    background-color: var(--bg-dk);
  }

/* LINKS and HIGHLIGHTED THEME TEXT */
  .decorated,
.autolinked,
.chat-line--integration .chat-line__body a,
.formatted_content a:not(.unstyled):not(.attachment__toolbar):not([contenteditable=false]):not([data-trix-attachment]),
.formatted_content a .mentionable-person,
.recordable--todoset .todolist__permalink,
.ui-datepicker-today a,
.project-index__header,
.project-index__description,
.latest-activity__project,
.calendar-grid__items,
.todo__note,
.todo__assignee,
.todo__date,
.boosts-report__date>span,
.boosts-report__date>span,
.txt--uncolor,
.project-list__group-header
  {
    color: var(--text-primary) !important;
  }
/* LIGHT BG, LIGHT TEXT, DARK BORDER */
  .btn,
.search__reset,
.report-menu__item--selected,
.input,
.nav-menu__sheet,
.message .boosts,
.document .boosts,
.recordable .boosts,
.boosts-report .boosts,
.boost,
.todos-form__date,
.btn--arrow-top-icon,
bc-autocomplete:not([multiple]) .autocompletableFDSFSD
  {
    color: var(--text-lt) !important;
    background-color: var(--bg-lt) !important;
    border-color: var(--bg-dk) !important;
  }
/* MEDIUM BG, MEDIUM TEXT, DARK BORDER */
  .card,
.options-menu__content,
.input--select,
bc-autocomplete .autocompletable,
.btn__keyboard-shortcut,
.panel--perma,
.panel--project,
.prompt,
.recordable,
.report-menu__item:hover:not(.report-menu__item--selected),
.card__link,
.boosts-report__card,
.chat__header--profile,
.recording-breadcrumbs,
.chat__header,
.chat-line__bubble,
.messages-table td
  {
    background-color: var(--bg-md) !important;
    color: var(--text-md) !important;
    border-color: var(--bg-dk) !important;
  }
/* LIGHT BG COLOR */
  .checkbox__button,
.options-menu__action:hover,
.tabs__tab--activ
  {
    background-color: var(--bg-lt) !important;
  }
/* MEDIUM BG COLOR */
  .miniaturize-cards .card--folder,
.card-grid--miniaturized .card--folder,
.card__content,
.boosts-report__date>span,
.search-result--selected,
.break>span,
.reading--show-actions,
.reading__actions,
.thread-entry--no-background,
.list--tableview li,
.nav__recent:hover,
.nav__my-stuff:hover,
.reading:hover,
.sheet
  {
    background-color: var(--bg-md) !important;
  }

/* SLIGHTLY DARKER VARIATION OF MEDIUM BG COLOR */
  .card::before,
.thread-entry,
.sheet--flat
  {
    background-color: var(--bg-md-2);
  }
/* DARK BG COLOR */
  .nav__main,
.ui-datepicker,
.ui-datepicker-header,
.break--on-background>span,
.custom-radio__button
  {
    background-color: var(--bg-dk) !important;
  }

/* THEME COLOR BG */
  .action-sheet__content,
trix-editor .attachment__toolbar
  {
    background-color: var(--bg-primary) !important;
  }

/* LIGHT TEXT COLOR */
  .tabs__tab:hover,
.tabs__tab--active,
.calendar-grid__month,
.calendar-grid__weekday,
.todo .todo__unassigned-unscheduled,
.copy-to-clipboard--for-search-result .copy-to-clipboard__button,
.options-menu__action:hover,
.tabs__tab--activ,
.nav__recent:hover,
.nav__my-stuff:hover,
.reading:hover
  {
    color: var(--text-lt) !important;
  }

/* MEDIUM TEXT COLOR */
  .reading__metadata,
.attachment--preview .attachment__caption,
.attachment:not(.attachment--preview) .attachment__caption>.attachment__size,
.attachment:not(.attachment--preview) .attachment__caption .attachment__attributes,
.floating-placeholder__label,
.recordable--todo .todo__header .checkbox__text,
.card__content>footer,
.scheduled-event__metadata,
.metadata-color,
.txt--very-subtle,
.txt--subtle,
.chat-line__timestamp,
.todo-progress__ratio,
.thread-entry__time
  {
    color: var(--text-md) !important;
  }

/* DARK TEXT COLOR */
  .message__date a, .message__date .link_button
  {
    color: var(--text-dk);
  }

/* BORDERS */
  table,
.todo-ledger__column,
.todo-ledger__header,
.card--app .card__header,
.messages-table--for-card,
.latest-activity::before,
.tabs,
.tabs__tab,
.calendar-grid__cell,
.calendar-grids,
.schedule-day,
.schedule__load-more,
.thread__subscriptions,
.list-picker-item+.list-picker-item,
.checkbox__button,
.schedule-card__agenda-view .schedule-month,
.recordable__archived-children,
.todos-form,
.schedule-month,
.questionnaire__question,
.card-inbox__forward,
.perma__header,
.list--tableview .list__action,
.notice--yellow,
.options-menu__action+.options-menu__title,
.message__attribution,
.tabs__tab:hover:not(.tabs__tab--active),
.custom-radio__button
  {
    border-color: var(--bg-dk) !important;
  }

  .custom-radio__input:checked+.custom-radio__button
  {
    border-color: var(--text-primary) !important;
  }

/* REMOVE SHADOWS */
  .panel--perma,
.panel--project,
.project-avatars__edit::after,
.tabs__tab--active,
.card--torn,
.recording-breadcrumbs,
.ui-datepicker,
.sheet--shadowed
  {
    -webkit-box-shadow: none;
    box-shadow: none;
  }

  .card--app .card__description
  {
    text-shadow: none !important;
  }

/* RICH TEXT EDITOR */
  trix-toolbar,
.trix-contained-input,
trix-toolbar .trix-dialog.trix-active.trix-dialog--link .trix-dialog__link-fields input[type=url], trix-toolbar .trix-dialog.trix-active.trix-dialog--link .trix-dialog__link-fields input[type=button]:last-of-type
  {
    color: var(--text-light) !important;
    background-color: var(--bg-dk);
    border-color: var(--bg-md);
  }

  .trix-contained-input a
  {
    color: #1b6ac9	!important;
  }

  .trix-contained-input .attachment--preview .attachment__caption
  {
    color: #6d6d6d !important;
  }

  trix-toolbar .trix-dialog.trix-active
  {
    background-color: var(--bg-md);
  }

  trix-toolbar .trix-dialog.trix-active.trix-dialog--link .trix-dialog__link-fields input[type=button],
trix-toolbar .trix-button.trix-button--icon.trix-active
  {
    background-color: var(--bg-primary);
    border-color: var(--bg-md);
  }

  .formatted_content pre
  {
    background-color: rgba(255,255,255,0.035);
  }

  .sticky-trix-toolbar
  {
    border-color: var(--bg-lt);
  }


/* INPUT PLACEHOLDER TEXT */
  ::-webkit-input-placeholder
  {
    color: var(--text-md) !important;
  }

  :-ms-input-placeholder
  {
    color: var(--text-md) !important;
  }

  ::-moz-placeholder
  {
    color: var(--text-md) !important;
  }

  ::-ms-input-placeholder
  {
    color: var(--text-md) !important;
  }

  ::placeholder
  {
    color: var(--text-md) !important;
  }

/* CSS ARROW PSEUDO ELEMENT and OTHER */
  hr,
.expanded .options-menu__expansion-toggle::after
  {
    border-bottom-color: var(--bg-md) !important;
  }

  .expanded .options-menu__expansion-toggle::before
  {
    border-bottom-color: var(--bg-dk) !important;
  }

  .break::before
  {
    border-bottom-color: var(--bg-lt) !important;
  }

  .break--on-background::before
  {
    border-bottom-color: var(--bg-lt) !important;
  }

  .nav-menu.expanded::after
  {
    border-bottom-color: var(--bg-lt) !important;
  }

/* OTHER EFFECTS */
  .card__content>footer
  {
    background: -webkit-gradient(linear, left top, left bottom, from(rgba(255, 255, 255, 0)), color-stop(30%, var(--bg-md))) !important;
    background: -o-linear-gradient(rgba(255, 255, 255, 0), var(--bg-md) 30%) !important;
    background: linear-gradient(rgba(255, 255, 255, 0), var(--bg-md) 30%) !important;
  }

  .calendar-grids__nav
  {
    opacity: 0.75 !important;
  }

  .help-button__icon
  {
    -webkit-filter: invert(1) !important;
    filter: invert(1) !important;
    background-color: #ff7100 !important;
  }

  .boosts-report__date,
.card--torn::before,
.card--torn::after
  {
    background: none !important;
  }

  .thread-entry__avatar,
.chat-line__avatar .avatar
  {
    -webkit-box-shadow: 0 0 0 1px var(--bg-md), 0 0 0 5px var(--bg-md) !important;
    box-shadow: 0 0 0 1px var(--bg-md), 0 0 0 5px var(--bg-md) !important;
  }

  .notice--yellow
  {
    background-color: #54524a !important;
  }

  .options-menu__content,
trix-toolbar .trix-dialog.trix-active
  {
    -webkit-box-shadow: 0 0 4px black, 0 5px 20px black;
    box-shadow: 0 0 4px black, 0 5px 20px black;
  }
.btn.btn--client-show {
    background-color: var(--bg-client-show) !important;
    color: var(--text-client-show) !important;
}
.todolist-group__header .break::after {
    background: linear-gradient(to right, rgba(255,255,255,0) 0%, transparent 100%)
}

.action-sheet__expansion-toggle {
    border-color: var(--text-dk) !important;
}

/* LOGO */
.nav__link--accounts {
    filter: invert(50%)
}

/* TRACKEDHQ */

#viewing_board .board-card {
    background-color: black;
    color: #ddd;
}
#viewing_board #button_bar {
    background-color: black!important;
}
#viewing_board .column-content {
    scrollbar-width: none;
}
#viewing_board .list_name {
    color: white;
}
#viewing_board .add_new_todo_show_form {
    color: black;
}
.tracked_card {
    background: var(--bg-primary);
}
.tracked_container_text {
    color: var(--text-md);
}
.panel.panel--project div.break center span {
    background-color: black !important;
    color: var(--text-lt);
}

Great work, thanks so much! I wonder why they still haven't come up with their own dark theme :-D

Thank you very much for creating this. Viewing traditional all white background sites and apps is down-right painful to my eyes. However, I can not figure out where Basecamp 3 allows me to edit or add a new css file. PLEASE HELP! I am using Basecamp version 2.2.4 on Windows 10. Thanks in advance -Bdevn

@Bdevn, it's not a default Basecamp functionality. This Gist changes Basecamp's styles, but you have to load them from somewhere.

I'm using this Stylus Chrome extension. Just copy and paste this code into the extension and turn it on for the basecamp.com domain.

OMG! Thank you much for the speedy reply igorlanko! The theme looks awesome and goes well with the overall theme on my computer. Great work!!!