Please use keyboard shortcuts for YouTube :
f => Toggle full-screen
t => Toggle theater mode (expand the playing area)

When in full-screen mode, or the player is in focus, then following shortcuts also work :

down arrow => Decrease volume by 10%
up arrow => Increase volume by 10%
space => Toggle play/pause

0 => Seek to the beginning of the video
1 => Seek to 10% of the video time (if video is of 55 minutes, then pressing 1 would take you to 5.5 minute marker)
2 => Seek to 20% of the video time
Same for 3, 4, 5, 6, 7, 8 and 9

left and right arrows go 5 seconds in the direction of the arrow

If the video is "paused" (and video-player is in focus) then you can move back and forward "1 frame at a time" using comma (,) and full stop (.)

I have set "Ctrl + Shift + Space" as global shortcut (in Ubuntu) to play/pause from anywhere, even when YouTube (or any other "audio/video player") is in the background