Add simple Daily Note Themes Editor

[Lauloque]

Add a simple theme editor that shows a preview of all themes, and lets the user customize the colors with a real time preview.
Once satisfied with the result, the user can hit a button to copy the new css code, and paste it in place

Demo:

Further development ideas for the brave ones (probably not me):

• Make the editor fetch the colors from the css file
• Make the editor update the css file with new colors
• Add a way to reset to the repo's default colors.