Here's a great overview of how themes work and how to change DayBack's fonts, colors, and backgrounds with CSS:
Using 3rd Party Editors
You may find working in a actual code editor easier if you're making a lot of changes to either the css files in DayBack or the translation files. Copy the text you're editing out of DayBack, paste it into your favorite editor, make your changes and then copy and paste it back.
Editors we like include Coda (for Mac) which has inline color pickers--always handy...
...and Sublime Text (all platforms) which offers great code completion and "multiple cursors" which are an absolute joy (shown with "next-skip" here).
Important Note on Smart Quotes:
Smart quotes (“ ”) are not compatible with CSS strings and will render the values invalid. When editing CSS themes in FileMaker, make sure to disable Smart Quotes (File Options -> Text -> Use smart quotes). Your opening and closing quotes should look identical in the editor (" ").