Use fixed background/color for inputs and textareas

[owi92]

Closes #932

I noticed that disabled textareas also use a slightly greyed-out font-color, so I added this effect to disabled inputs as well.

[LukasKalbertodt]

Mh, it looks good but I'm thinking whether it could look even better. You chose a flat look with the inputs having the same background color as the background. If we compare to GitHub:

There the input is slightly darker than the background. I'm not sure about light mode, but in dark mode I kinda like it. But of course, you are already using the darkest color we have :D So before we somehow introduce a darker color... would we even want to? Does anyone else agree even darker is nice?

Example (via dev tools):

vs

What do you think?

[owi92]

I think darker is nice but I'm not sure if we really want to add another color for that. Also not sure about light mode, but I'm leaning towards keeping it flat on that side.