feat: add custom theme support

[leocavalcante]

Summary

• add config-driven custom themes with built-in preset inheritance, UI/Markdown color overrides, and optional syntax theme selection
• allow --theme / theme to select built-in presets, named inline custom themes, or external TOML theme files
• document external theme-file usage while keeping config.toml compact and preserving ocean as the default

Validation

• env -u LEAF_EDITOR -u VISUAL -u EDITOR cargo test
• cargo clippy --all-targets --all-features -- -D warnings
• cargo build --release

Notes

• Squashed to a single commit and removed the Copilot co-author trailer.
• Commit is SSH-signed with lc@leocavalcante.com and GitHub reports it as verified.

[RivoLink]

Hi @leocavalcante
Thanks for your interest in the project and for your PR — I really appreciate it.

A couple of requests:

• External theme file: The inline [themes.gruvbox] section makes config.toml quite long. Could you export the custom theme into a separate file (e.g., gruvbox.toml) and reference it in config.toml with something like theme = "path/to/gruvbox.toml"? This would keep the main config clean. If that's too much for this PR, no problem — I can open a follow-up issue for it.
• Squash into a single commit and please remove Copilot as co-author.
• Signed commit: GPG signing is required on main. If you can sign it, great. If not, no worries — I'll handle it and preserve you as author.

GPG signing docs : https://docs.github.com/en/authentication/managing-commit-signature-verification/signing-commits

[leocavalcante]

Thanks for the review!

I addressed the requests:

• Kept config.toml compact with ocean as the default.
• Added support for external TOML theme files, resolved relative to config.toml, so users can set e.g. theme = "gruvbox.toml" without inlining a long [themes.*] section.
• Documented the external theme-file format without committing my personal Gruvbox theme file to the repository.
• Squashed the PR to a single commit.
• Removed the Copilot co-author trailer.
• Updated the commit author/committer email to lc@leocavalcante.com.
• Re-signed the commit using SSH signing; GitHub reports it as verified.

Validation passed:

• env -u LEAF_EDITOR -u VISUAL -u EDITOR cargo test
• cargo clippy --all-targets --all-features -- -D warnings
• cargo build --release

[RivoLink]

Thanks @leocavalcante!
The squashed commit is clean, the external theme file approach works well, and the tests are solid.
Merging, great contribution!

[RivoLink]

Hi @leocavalcante
Can you rebase your branch from main to validate the CI checks?
I added some fixes to the CI checks.

[leocavalcante]

Rebased onto the latest main with the CI fixes and force-pushed the signed single commit.

Current head: e52de10

Local validation passed again after the rebase:

• env -u LEAF_EDITOR -u VISUAL -u EDITOR cargo test
• cargo clippy --all-targets --all-features -- -D warnings
• cargo build --release

GitHub reports the commit signature as verified. I’m waiting for Actions to attach/rerun the PR checks on the new head.

[RivoLink]

Perfect, thank you again!