Tuuba being barely visible and cringe are intentional. I don't have any problem with the text so can't say anything about readability.

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Removing Art subforum
Move the threads in Art under Other media or make the Art subforum into subforum within subforum.

Combine G&R with Other Media
Agree with this. Also the Games subforum which tupsu was suggesting would be good idea.

Removing Lewd subforum
I'm against this. In my opinion we should have a separate space for NSFW or ban all NSFW content entirely. Considering the current userbase I think the former is better option.

Removal of ITT
I don't think this makes any difference either way, short term it might increase topics in OT but eventually another similar thread will start dominating the OT subforum.

To be honest i don't really know how much activity you are expecting. If you apply the 90-9-1 principle to the size of Tuuba-community activity levels seem pretty standard.
The best way to increase activity would be getting new members and as far as I know there's no active promotion going on.