[Ready for Review] Implement dark theme option

[helendduncan]

Expected behaviour

On first load of the page detects users' system theme, and sets the theme to the corresponding option (light/dark)

Button added in Nav bar to toggle between light and dark modes. If selected, the selected theme is stored in Local Storage.

Some re-defining classes in the css file to match the bootstrap defaults for more consistent switching from light to dark mode, and added a "grey" copy of the flowchart on the help page.

Things that perhaps need review/thought.

• [ ] "Toggle Theme" should the wording in the nav bar be more explicit (switch light/dark mode) or in fact be a toggle?
• [ ] On the home page server/apps/main/templates/main/home.html there are inconsistencies with the background/foreground for the "About AutSPACEs" section when compared to the shared stories carousel section (what turns light/dark) but the text colour in the "About AutSPACEs" section doesn't look particularly contrasting when the background is dark. Needs someone with design veto to decide.

Closes #604

[github-actions[bot]]

Coverage report

This PR does not seem to contain any modification to coverable code.

[helendduncan]

This is a page on dark mode

This is also a page on dark mode

Query @gedankenstuecke - which is the preferred behaviour? Dark text in the colour header or for it to stay as white when you switch to dark mode? My initial intuition is that the text should be dark on dark mode?

[gedankenstuecke]

Thanks for all the work on this @helendduncan!

I think typically dark mode is similar to "inverting" the colours, with mostly dark-ish backgrounds and light grey/off-white text on top of it. In that sense the second one is probably closer to that.

The top one might have the problem of being too low-contrast depending on accessiblity needs?

tl;dr: if non-dark mode is already "white text on darker background" we might want to keep it that way in dark mode too :)

[gedankenstuecke]

Also re-worded the navbar item a bit, happy to have it merged now!