Preview improvements
This is a continuation of #126. I've added a couple more changes around theming / styles.
Relevant changes include:
- Responsive preview pane following the same breakpoints that the app uses
- Encapsulation of theme / style handling in a small dedicated
Theme
class - Support for HighContrast and HighContrastInverse, besides the existing Adwaita, Adwaita-dark, Arc, Arc-Darker and Arc-Dark
- Removal of hard-coded colors from MarkupBuffer
Regarding 2, this would make it easy to support more and custom themes as mentioned in #123. Adding more default themes is trivial, plus if we provide a setting to the user to pick custom CSS file (for the preview, editor, or both), it's easy to support a custom theme as well.
Additionally, after your question in #126, I've decided to automate the color palette generation and included it in 152db98f (I did this by hand, but it's the same formula). I am unsure if this belongs in the repo, or if that is the place where you'd put it, so feel free to change.