feat(cli): support adaptive theme syntax in --theme argument

[pamburus]

Summary

The --theme argument now supports adaptive theme syntax, allowing different themes for dark and light modes directly from the command line.

Changes

Adaptive theme syntax for --theme

The --theme flag now accepts two forms:

• Fixed: --theme my-theme — uses the same theme regardless of mode
• Adaptive: --theme "dark:one-dark,light:one-light" — uses different themes depending on the resolved mode

Previously, the --theme flag only accepted a plain theme name and always overrode the config with a fixed theme. Adaptive themes configured in config.toml were lost when --theme was specified. Now, the CLI argument supports the same expressiveness as the config file.

The default value from the config is now shown in --help output.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.43%. Comparing base (c434cc2) to head (728c987).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #503      +/-   ##
==========================================
+ Coverage   77.27%   77.43%   +0.16%     
==========================================
  Files          31       31              
  Lines        4140     4166      +26     
==========================================
+ Hits         3199     3226      +27     
+ Misses        941      940       -1     

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