I switched to windowed mode, and then back to fullscreen mode, and now to resolution is very poor, like 720p. (Windowed mode was also like 720p, not sure of DPI scaling.)
Anyway, the letters are now very blurred. How to fix? Is there a way to change resolution anyway?
Hardly playable right now 
Unity games store their current window resolution in the Windows registry, so you can open regedit (with the game closed) and manually adjust the resolution to whatever you want (including resolutions greater than what the game would otherwise let you do, like 1440p or 4K): Where does the display resolution dialogue box save it's settings? - Questions & Answers - Unity Discussions
In this case that would be at HKEY_CURRENT_USER → Software → Nekoday → Christmas Tina -泡沫冬景-
Make sure when editing them you have Decimal and not Hexadecimal selected, or your window might end up slightly larger than expected.
Thank you very much. Now it works cleanly. Keeping at 1920x1080 for now. (My actual screen is max 2880x1800.)
Switching back and forth to windowed mode also doesn’t break anything.
No wonder that uninstall/reinstall, and deleting local files (not sure if I could find them all), don’t help anything.
