After clicking the play button in the video player screen, you will see a gear icon in the bottom right corner. Click on that and then select your preferred video quality.
The video quality starts with Standard Definition (SD) 480p. If that isn't sharp enough for your screen, you can select High Definition (HD) either 720p or 1080p.
For small devices or if you have low bandwidth you can also downgrade the quality to 360p or 240p.
With the other icons you can mute/unmute, set volume, or enter fullscreen mode.
Newer Mac: Control-⌘-F
Older Mac: read this
(repeat to exit)