If you're having issues getting TETR.IO PLUS to work, please consider asking for support in the relevant Discord server linked from the wiki homepage on Gitlab.

Having the main TETR.IO PLUS window fail to open sounds like an unusual issue - are you sure you're on the TETR.IO website and clicking the extension icon in the far right of the address bar?

The grid isn't a sprite by itself but appears to be drawn using primitives, which means I can't replace it as easily as with block skins or backgrounds. With TETR.IO's upcoming character update, this is probably going to change since the game is adding character board backgrounds, at which point this might be possible.