Thanks a lot for trying! Contrast was something I absolutely wasn't thinking about, and I can totally see how that could be an issue, thank you for the feedback! I've made jam games with fullscreen capabilities in the past, but if I'm being totally honest I just forgot about it this time haha. It doesn't help that I don't have a lot of experience with supporting different kinds of resolutions or aspect ratios with GameMaker, but that's certainly something I'm going to work on in the future :D
Way back, when I used Game Maker (around 2010), there was an option to "allow fullscreen" or something like this, and you could press F10 or something, to switch between fullscreen and non-fullscreen, you didn't have to tweak anything. Is it not in the new versions anymore?