Test-theme1

[2003Aditya]

Summary

1. This PR implements a Light Theme for the TUI and refactors theme handling to be flexible and extensible. Key changes:
2. Introduced lightTheme() and made the original the official darkTheme().
3. Added a --theme flag to diffium watch to switch the base theme.
4. Updated the Theme struct with AddBgColor and DelBgColor and added AddLine/DelLine methods to ensure added/deleted lines have visible background contrast.
5. Ensured .diffium/theme.json overrides still correctly layer on top of the selected base theme.

Linked Issue(s)

Closes #45

Checklist

[ ] I am assigned to the linked issue(s)

[ ] I wrote tests or updated existing tests

[x] I ran go build and go test ./...

[ ] I updated docs/README if needed

[2003Aditya]

Hey! @kubeden can u please help me with how u want light theme to be?? So , i can make this issue complete

[kubeden]

Hey. From looking at your code, the diffium watch --theme light is a good foundation, but I noticed it does not work. Generally, I would like Diffium to have the possibility for different themes, separately from the terminal theme the users have. Let me know if you can do that

Screen.Recording.2025-10-05.at.12.21.45.mov

[2003Aditya]

@kubeden actually i had changed some color to show you, see these images both image has red color but with alot of difference , one is theme-light and another one is the default one .
so if u could give me some example exactly how you want these themes to be look i would work on that .