Ignoring what looks to be a very nice game and game concept.
I agree, with these settings, the game looks really bad on Itch.
Adventuron is meant to be played in full screen mode on Itch, rather than the tiny display window they give you otherwise. Off Itch, you can play in a browser, so the recommendation is always to test your game in full screen mode. I really need to see your theme settings, and I'll mess about myself.
Rest assured, I can see the problem now, so I will provide you with the config changes or bugfixes you need.