Google really likes icons that are just outlines. After the multicolored makeover for all of Google's service app icons, it redid the YouTube interface, replacing the standard look with line art. Now that shift has made its way to the desktop/web version of YouTube. So yeah, that's a thing.
The revised icons appeared on the left side menu of YouTube today—Archive.org shows that the older icons were active as recently as yesterday. As with all Google interface shifts, it might take a while for it to roll out to everyone. This brings the web version of YouTube in line with the app changes made back in October and November of last year.
Old icons on the left, new on the right. Slide to see the difference.
The new YouTube icons follow some of the interface changes made to other Google services, like Gmail and Messages. The reaction from users has been generally negative, with some complaining that the new icons are harder to distinguish from each other.