Customizable themes

Implements #41 (closed), needs review and testing.

• Change how base16 rules are used globally (base01 panel-headings -> base02 for example)
• Add a theme editor in settings menu, complete with a preview
• Remove the usage of base16-theme.css files in favor of themes consisting of only 4 main colors
• Add a couple preset themes in styles.json
• Make themes persist