Theme exports omit default editor and Markdown colours

Export Theme in Settings > Appearance omits unchanged editor and Markdown preview colours from the saved theme file. Only customised values are included in those sections, so the file is not a complete record of the selected colours.

Export every supported colour for both the light and dark variants, including defaults, while preserving customisations. This makes exported themes complete and keeps their appearance consistent if Studio's built-in defaults change in a later version. Existing theme files should continue to import correctly.