Tampermonkey / Greasemonkey script for night mode / dark theme in Telegram web (scroll down for instructions)

telegram_dark_mode.user.js

// ==UserScript==
// @name         Telegram dark theme / night mode
// @namespace    https://github.com/digitalheir/
// @version      1.0
// @description  Dark theme / night mode for Telegram Web
// @author       Maarten Trompper <maartentrompper@freedom.nl>
// @match        https://web.telegram.org/*
// @grant        GM_addStyle
// ==/UserScript==
GM_addStyle('body { color: #fff;');
GM_addStyle('h1, h2, h3, h4, h5 { color: #fff; }');
GM_addStyle('.md_modal_head { background: #5f5f5f; }');
// GM_addStyle('.im_message_body { background: #4f4f4f; }');
// GM_addStyle('.im_message_out .im_message_body { background: #4f4f4f; }');
GM_addStyle('.im_page_split { background-color: #333; }');
GM_addStyle('.page_wrap { background-color: #232323; color: #fff;}');
GM_addStyle('.im_page_wrap { background-color: #232323; box-shadow: 0px 1px 0 #2b2b2b; border-left: 1px solid #2b2b2b;border-right: 1px solid #2b2b2b;border-bottom: 1px solid #2b2b2b;}');
GM_addStyle('.im_dialogs_col_wrap { border-right: 2px solid #2b2b2b;}');
GM_addStyle('.tg_head_split { background-color: #3c3c3c;}');
GM_addStyle('.dropdown.open .tg_head_btn, .tg_head_btn:hover { background-color: #4e4e4e;}');
GM_addStyle('.im_dialogs_scrollable_wrap .active a.im_dialog:hover, .im_dialogs_scrollable_wrap .active a.im_dialog_selected{ background-color: #4e4e4e;}');
GM_addStyle('.im_dialog_peer { color: #eee;}');
GM_addStyle('.im_dialogs_scrollable_wrap .active a.im_dialog { background-color: #3c3c3c;}');
GM_addStyle('.im_dialogs_scrollable_wrap a.im_dialog:hover, .im_dialogs_scrollable_wrap a.im_dialog_selected { background-color: #444444;}');
GM_addStyle('.form-control { background-color: #333; }');

// badges
GM_addStyle('.im_dialog_badge { background: #1e7729;}');
GM_addStyle('.im_dialog_badge_muted { background-color: #444444;}');

// scroll bar
GM_addStyle('.im_history_col .nano > .nano-pane > .nano-slider, .contacts_modal_col .nano > .nano-pane > .nano-slider, .sessions_modal_col .nano > .nano-pane > .nano-slider, .stickerset_modal_col .nano > .nano-pane > .nano-slider, .im_dialogs_modal_col .nano > .nano-pane > .nano-slider { background: rgba(66, 66, 66, 0.5);}');
GM_addStyle('.im_dialogs_col .nano > .nano-pane > .nano-slider { background: rgba(220, 220, 220, 0.3);}');

// selected message
GM_addStyle('.im_message_selected .im_message_outer_wrap { background: #404040; }');
GM_addStyle('.im_history_select_active .im_message_outer_wrap:hover {background: #464646;}');

// make links light blue
GM_addStyle('.im_message_mymention { background: #333333; }');
GM_addStyle('.im_message_author, .im_message_fwd_author { color: #c1e2ff;}');
GM_addStyle('a { color: #c1e2ff;}');
GM_addStyle('a:focus, a:hover { color: #d8edff; }');
GM_addStyle('a.im_dialog .im_dialog_chat_from_wrap, a.im_dialog .im_short_message_media, a.im_dialog .im_short_message_service  { color: #c1e2ff;}');
GM_addStyle('a:focus, a:hover { color: #c1e2ff; }');
GM_addStyle('.im_message_reply_author { color: #c1e2ff; }');

// unread messages banner
GM_addStyle('.im_message_unread_split { background: #444; color: #cacaca;}');

// search field
GM_addStyle('.im_dialogs_search_field { border: 1px solid #333; background-color: #333;}');
GM_addStyle('.im_dialogs_search_field:active, .im_dialogs_search_field:focus{ border: 1px solid #333; background-color: #333;}');
GM_addStyle('.form-control { color: #fff; }');
GM_addStyle('.im_dialogs_scrollable_wrap a.im_dialog_searchpeer, .im_dialogs_scrollable_wrap a.im_dialog_searchpeer:hover, .im_dialogs_scrollable_wrap a.im_dialog_searchpeer_selected { background: #4f4f4f; }');

// input field
GM_addStyle('.composer_rich_textarea:focus, .composer_textarea:focus { box-shadow: 0 2px 0 0 #616161; }');
GM_addStyle('.composer_rich_textarea { overflow-y: hidden; }');
GM_addStyle('.im_record { background: #4f4f4f; }');
GM_addStyle('.composer_autocomplete_option_active a.composer_command_option .composer_command_desc, a.composer_command_option:hover .composer_command_desc { color: #fff; }');
GM_addStyle('.im_record_bg { position: absolute; margin: -3px 0px 0px -9px; padding: 5px 2px 5px 5px; width: 35px; height: 35px; border-radius: 50px; overflow: hidden; background: #333; transition: background-color linear 0.2s;}');
// popup
GM_addStyle('.modal-content { background-color: #2f2f2f;}');
GM_addStyle('a.tg_checkbox, p.tg_checkbox { color: #fff;');

// dropdown menu
GM_addStyle('.dropdown-menu { background-color: #404040; }');
GM_addStyle('.dropdown-menu>li>a:focus, .dropdown-menu>li>a:hover { background-color: #545454; }');
GM_addStyle('.tg_head_logo_dropdown .dropdown-menu > li > a:hover { background: #545454; }')
GM_addStyle('.tg_head_logo_dropdown .dropdown-menu > li > a {color: #fff;}')
GM_addStyle('.tg_head_logo_dropdown .dropdown-menu { border-left-color: #383838; border-right-color: #383838; -webkit-box-shadow: 0px 1px 3px 0px rgba(78, 78, 78, 0.27); -moz-box-shadow: 0px 1px 3px 0px rgba(60, 75, 87, 0.27); box-shadow: 0px 1px 3px 0px rgba(39, 39, 39, 0.27); }')
GM_addStyle('.icon-new-group {-webkit-filter: brightness(0) invert(1); filter: brightness(0) invert(1);}');
GM_addStyle('.icon-contacts {-webkit-filter: brightness(0) invert(1); filter: brightness(0) invert(1);}');
GM_addStyle('.icon-settings {-webkit-filter: brightness(0) invert(1); filter: brightness(0) invert(1);}');
GM_addStyle('.icon-faq {-webkit-filter: brightness(0) invert(1); filter: brightness(0) invert(1);}');
GM_addStyle('.icon-about {-webkit-filter: brightness(0) invert(1); filter: brightness(0) invert(1);}');

// settings
GM_addStyle('a.tg_radio { color: #fff; }');

// chat dropwdown
GM_addStyle('.tg_head_peer_dropdown .dropdown-menu > li > a { color: #fff; }');
GM_addStyle('.tg_head_peer_dropdown .dropdown-menu { border: 1px solid rgba(64, 64, 64, 0.2); -webkit-box-shadow: 0px 0px 2px 0px rgb(45, 45, 45); -moz-box-shadow: 0px 0px 2px 0px rgb(45, 45, 45); box-shadow: 0px 0px 2px 0px rgb(45, 45, 45); }');
GM_addStyle('.tg_head_peer_dropdown .dropdown-menu > li > a:hover {background-color: ##545454;}');
GM_addStyle('.dropdown-menu>li>a {color: #fff;}');

// emoji
GM_addStyle('.composer_emoji_tooltip.noselect.composer_emoji_tooltip_shown { background-color: #252525; }');
GM_addStyle('.composer_emoji_tooltip{border: 1px solid #252525;}');
GM_addStyle('.icon-tooltip-tail { background: #252525; border: 1px solid #252525;}');
GM_addStyle('.md_modal_section_splitter { background-color: #303030; }');
GM_addStyle('.composer_emoji_tooltip_tab_shadow { width: 50%; height: 1px; background: #ffffff; -webkit-box-shadow: 0 1px 0 0 #ffffff; -moz-box-shadow: 0 1px 0 0 #53a9ea; box-shadow: 0 1px 0 0 #ffffff;}');
GM_addStyle('.composer_stickerset_title { color: #fff; }');
GM_addStyle('.composer_emoji_tooltip_tab_emoji, .composer_emoji_tooltip_tabs_stickers_active .composer_emoji_tooltip_tab_stickers {color: #fff;}');
GM_addStyle('.composer_emoji_tooltip_category i { filter: brightness(0) invert(1); }');
GM_addStyle('.tg_head_peer_dropdown .dropdown-menu > li > a:hover { background: #545454; }');

// sticker popup in text field
GM_addStyle('.composer_dropdown_wrap { background: #5f5f5f; }');

// commands
GM_addStyle('.composer_dropdown .composer_command_desc { color: #fff; }');
GM_addStyle('.composer_dropdown .composer_command_value { color: #252525; }');
GM_addStyle('.composer_dropdown li a:hover, .composer_dropdown li.composer_autocomplete_option_active a { background: #4f4f4f; }');

// mentions
GM_addStyle('.composer_dropdown>li>a { color: #c1e2ff; }');
GM_addStyle('.composer_user_mention { color: #fff; }');
GM_addStyle('.composer_dropdown_wrap { background: #4f4f4f; }');

// contacts
GM_addStyle('.contacts_modal_members_list a.contacts_modal_contact { color: #fff; }');
GM_addStyle('a.mobile_modal_action, span.mobile_modal_action { color: #fff; }');
GM_addStyle('.contacts_modal_members_list .active a.contacts_modal_contact, .contacts_modal_members_list .active a.contacts_modal_contact:hover, .contacts_modal_members_list a.contacts_modal_contact:hover { background: #454545; }');

// Message focus animation (click message from search)
GM_addStyle('@keyframes im_message_focus_fade { from { background-color: #212223; } to { background-color: rgba(242,246,250,0) } }');

GM_addStyle('.telegram_modal_logo{ filter: contrast(33) grayscale(); }');

1. Install Tampermonkey for Chrome or Greasemonkey for Firefox
2. Import script into Tampermonkey / Greasemonkey by clicking the "Raw" URL (should bring you to the installation screen) or copy-paste the source code
3. Go to https://web.telegram.org/

This is what it looks like

I LOVE THIS GIST
A very comfortable dark theme for the telegram web client. I've forked it, the only thing I've changed is the file extension (.user.js) so that greasemonkey recognizes it as user script and line 7

Thank you @kroeliebuschie. I did the same!

cheers! 🍻 I made it working in Greasemonkey 4: https://gist.github.com/ewerybody/9aaf6069734b4eda355405aca539d263
and darkened code things and emoji/sticker selection

Under //Input Field i added

GM_addStyle('.im_record_bg { position: absolute; margin: -3px 0px 0px -9px; padding: 5px 2px 5px 5px; width: 35px; height: 35px; border-radius: 50px; overflow: hidden; background: #333; transition: background-color linear 0.2s;}');

This made the microphone icon to be the same as the others feel free to use

I took this script and modified a bit https://github.com/sluongng/web-telegram-widescreen/blob/master/web-telegram-dark-widescreen.user.js

This makes the web-ui of telegram web to look a lot better on widescreen(1920 x 1080).
The width was hard coded (modified value from original CSS) thus probably need further modification for other res.