* feat: add per-mode theme overrides
allow [theme.custom.light] and [theme.custom.dark] blocks so custom
overrides can differ per resolved appearance when auto_switch flips.
precedence: theme defaults, then [theme.custom], then the block for
the active mode. configs without the new blocks resolve unchanged.
refs #837
* docs: complete per-mode theme documentation
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Co-authored-by: Ogulcan Celik <ogulcancelik@gmail.com>