YouTube cleanup. Makes masthead stay put at the top of the page instead of following you down. Embiggens theater mode player to be like an actual video player. Auto-expands YouTube comments. Manages skeleton theater.

YouTube Redesign Redesign.user.css

@-moz-document domain("youtube.com") {
/*
YouTube has a skeleton theater. Spooky scary skeletons lurk before the page fully loads.
YouTube, probably: http://www.prequeladventure.com/this/story2068.gif
*/
button[title="Play on TV"] { /*Chrome get this garbage out of here.*/
	display: none !important;
}
#search-form input#search { /*Chrome intelligently selects a dynamic default foreground color based on background color. Firefox doesn't.*/
	color: #ffffffe0; /*Not !important, since we're just supplementing a browser default.*/
}
html[style] {
	overflow-x: auto !important;
	overflow-y: scroll !important;
}
html[style] body {
	overflow: visible !important;
}
#masthead-container {
	position: static !important;
}
#page-manager.ytd-app {
	margin-top: 0 !important;
}
ytd-masthead.shell, /*SPOOKY*/
ytd-masthead > #container.ytd-masthead {
	height: 48px !important;
}
ytd-expander.ytd-comment-renderer > .ytd-expander {
	max-height: none !important;
	overflow: visible !important;
}
ytd-expander.ytd-comment-renderer > paper-button > .more-button {
	display: none !important;
}
#player.skeleton.theater, /*SPOOKY*/
ytd-watch-flexy[theater] #player-theater-container {
	height: calc(100vh - 48px) !important;
	min-height: 0 !important;
	max-height: none !important;
	overflow: hidden !important;
}
#player.skeleton.theater #player-api{ /*SPOOKY*/
	box-sizing: border-box !important;
	padding-top: 39px !important;
	background-color: black !important;
}
ytd-watch-flexy[theater] .html5-video-player:not(.ytp-fullscreen):not(.ytp-small-mode) {
	height: calc(100% - 39px) !important;
	overflow: visible !important;
}
ytd-watch-flexy[theater] .html5-video-player:not(.ytp-fullscreen):not(.ytp-small-mode) .annotation.iv-branding,
ytd-watch-flexy[theater] .html5-video-player:not(.ytp-fullscreen):not(.ytp-small-mode) .ytp-chrome-bottom {
	bottom: -39px !important;
}
ytd-watch-flexy[theater] .html5-video-player:not(.ytp-fullscreen):not(.ytp-small-mode) .ytp-tooltip.ytp-bottom,
ytd-watch-flexy[theater] .html5-video-player:not(.ytp-fullscreen):not(.ytp-small-mode) .ytp-settings-menu {
	margin-top: 39px !important;
	margin-bottom: -39px !important;
}
#player.skeleton.theater { /*SPOOKY*/
	margin-top: 48px !important;
}
ytd-watch-flexy[theater] .html5-video-player.ytp-small-mode:not(.ytp-fullscreen)[style*="right"][style*="bottom"] {
	width: 448px !important; /*Iridium corner video: embiggen.*/
	height: 252px !important;
}
#player.skeleton.theater .html5-video-player:not(.ytp-small-mode){ /*SPOOKY*/
	overflow: visible !important;
}
#player.skeleton.theater .html5-video-container video{ /*SPOOKY*/
	margin-top: -39px !important;
}
ytd-watch-flexy[theater] .html5-video-player:not(.ytp-fullscreen):not(.ytp-small-mode) .ytp-gradient-bottom {
	display: none !important;
}
ytd-sentiment-bar-renderer {
	padding-top: 0px;
}
#container.ytd-sentiment-bar-renderer,
#like-bar.ytd-sentiment-bar-renderer {
	height: 8px !important;
}
yt-formatted-string:focus {
	outline: none !important;
}
ytd-watch-flexy[theater] .html5-endscreen {
	height: 100% !important;
}
}

Thanks, always found the youtube bar scrolling with you to be an annoyance and was looking for a long time for a way to get it pinned in place. I had found this but kept looking since I did not have any addons installed to handle .css files and didnt wanna find 1 if I could help it but after hrs of searching I gave up and just installed Stylus so I could use this.