redesign follow-up for footer

I have these three changes to the footer:

1. The probably least controversial is to align the footer to the bottom of viewport instead of page content's bottom.
2. More involved is the colour change: I would argue that the footer is not supposed to elements with the highest contrast overall on each page, and being the only one with pure white or black background buttons. While not using buttons, in #169 (comment 1888849932) the footer also only had white on green text, giving a more balanced impression overall. This also ensures that the inline links have a hover colour.
3. (In the same patch as the colour change) There is a gap change. This purely to harmonise with the header. The footer had a button gap not used anywhere else in the website's design, being unusually narrow, making it look compressed.
4. I can see this change is being more controversial: I suggest removing the back to top button. This is a static website and not a web app. Thus, this simply works around a UX shortcoming of the used browser. I would argue to report this browser vendors and not to fill footers with such buttons.

Example page: https://postmarketos.org/edge/2024/02/05/gtk4-new-renderers/

before	after

Ignore the logo size and alignment. Playing with a lot of parts.