I made changed to login page (css, logo, etc.), and it all worked fine, but I have a problem with it because it all resets each time WordPress is updated to a newer version.
Is there way to prevent this from happening, or do I need to reapply the changes each time?
The biggest question to ask yourself is, where are you putting in your overrides?
The best way to override this is with a function that lives in either a custom plugin or your theme that is not tied to the Core. If you are editing the core CSS (for admin) then yes it will be overridden.
Here’s my go-to function set: