>>611
横着したら普通の方のデザインが崩れてたので訂正
:root {
--controller-height: 51px;
}
ytd-watch-flexy[theater] #full-bleed-container {
margin-bottom: var(--controller-height) !important;
}
ytd-watch-flexy[theater] {
z-index: 0 !important;
}
ytd-watch-flexy[theater] .html5-video-player {
overflow: visible !important;
}
ytd-watch-flexy[theater] .ytp-chrome-bottom {
margin-bottom: calc(var(--controller-height) * -1) !important;
}
ytd-watch-flexy[theater] #player-container::after {
content: '';
position: absolute;
width: 100%;
height: var(--controller-height);
margin-bottom: calc(var(--controller-height) * -1);
background: #000;
z-index: -1;
}

ついでにツールチップの位置を下げるやつ

.ytp-tooltip {
margin-top: var(--controller-height) !important;
}