[Enhancement] Add theming MVP

**Is your feature request related to the youtui app, or one of the helper crates (e.g ytmapi_rs)?** youtui **Is your feature request related to a problem? Please describe.** Add selectable themes to youtui - per roadmap **Describe the solution you'd like** MVP: Themes hardcoded in app Exact theme is selected in config **Describe alternatives you've considered** Theme selctable in app: requires an in app commandline which Im not sure we want Theme declaration by user would be beyond MVP Theme selection at commandline would be tech debt to remove later **Additional context** None