improve the look of the sign-in page in dark mode

[DinCahill]

The code is a bit janky, but the screenshots are pretty.

Before:

After:

Light mode for reference:

[timche]

I've noticed this bug too, but it's inconsistently happening. I wonder what the cause of it is actually.

[timche]

I'm waiting with merging this, because I'm a little worried of the hardcoded classes which are prone to change as they are dynamic and afaik the sing-in page looks different depending on the browser. There is a slight chance this fix may not work for the user agents on Windows and Linux and has to be verified.

[timche]

As assumed, the sign-in page is broken depending on the user agent. I'm using macOS.

With Use User Agent Fix disabled:

With Use User Agent Fix enabled:

[timche]

I'll merge this for now as it would fix the sign in page for custom user agents at least.