Support dark style
Intended to fix #2192.
- Adds several palette colors to be used elsewhere
- Switches a couple of JPG references to PNGs

- [ ] Button styles
- [x] Brand logomark backgrounds
- [x] Get Involved tweaks
- [x] Open Source chart and logo colors
- [ ] Store popups are white-on-white
- [ ] Language popover is always white
Is this still open? I’d love to take the remaining tasks on. Any color preferences for buttons? or are those color vars already declared?
@SeanRParker yes, this would still be great to get wrapped up! The main issue with the button styles is that the strokes/shadows are meant for a white background, so they look weird on dark backgrounds. We can use transparency (like I did here: https://github.com/elementary/blog-template/pull/11) to work around that, or we could provide separate button styles for the dark style; whichever makes the most sense.
I don't typically run php so set up took me a bit on my macbook. I have things up though and the button looks good. I'd start on the #choice-buttons, but I got to get to bed. I hope to look at those next.

