More theme fixes

[Samir-Rashid]

This PR contains a series of a few independent fixes.

1. 3d308f4 I found one more css bug in the dark theme, relating to how <figcaption>s are shown
2. 0e160dc Fixes default theme should match user preference #2953. By default, the website will respect the dark or light mode preference expressed by the user's browser. The preference can still be overridden by the toggle button. I tested this by deleting the stored value in browser Local Storage, and the theme now follows the light/dark mode of your browser (no toggle clicking needed).
3. 259ee5a the new autoscroll dep had two bugs: it didn't have enough padding for the masthead, and it also did not update the URL when you click on a link like #scroll-to-id. These problems are now fixed, making it easier to share sections of webpages.
4. 1de3d66 Fixed image carousel popup for image links and added image expanding popup for normal images.

[rjzupkoii]

Excellent work, thanks for the updates to the theme!