If you do not need coloured text on your web page (due to coloured diagrams, colour coding, etc) then please do not specify colours at all. (If you do need colours, then ensure that all of the colours are specified.)

I hate it when the font color is set but not the input background. On computers with dark themes this can result in black on black text. There are workarounds in Firefox and I use them but this cost me more time than necessary.

There's no real consensus. Try this site: https://contrastrebellion.com/

Sure, but there's a whole spectrum between slightly darker grey on slightly lighter grey, and straight up #FFF on #000 or vice versa.

If I have to adjust my monitor brightness to be okay with reading something there's a major issue.

> If I have to adjust my monitor brightness to be okay with reading something there's a major issue.

Yes, but the issue is with your monitor brightness or your ambient lighting, not the content. I tune my monitor for use at night with the lights on and as a result, #000 on #FFF is perfectly comfortable at all times. There's rarely a good reason to use a PC in complete darkness and I can't think of any reasons to do it in your own home.