[idea] Support custom light/dark theme pairs with automatic switching

[aspiers]

idea / problem

Herdr can automatically switch between built-in themes using dark_name and light_name, and [theme.custom] can override palette tokens. However, there is only one shared custom overlay, so it cannot represent a pair of customized light and dark themes.

For example, I want to start from Catppuccin Mocha and Latte and adjust their surface colors independently to improve active-row contrast. A suitable Latte override is not suitable for Mocha, so the shared overlay defeats automatic switching.

requested change

Please support independently defined custom themes which can be referenced by dark_name and light_name, or simply give me the green light on the idea and I can submit a PR.

I do not have a strong preference between named inline theme tables and external theme files. The important part is being able to construct two complete custom palettes and use them as an automatically switched pair.

why you want this

I want to retain Herdr's automatic light/dark switching while making small, mode-specific adjustments to the Catppuccin palettes. Currently that requires either changing the managed config on every switch or maintaining a custom Herdr build.