theme: Properly merge `SyntaxTheme` styles to allow for partial overrides (#11911)

This PR improves the merging behavior for the `SyntaxTheme` such that
user-provided values get merged into the base theme.

This makes it possible to override individual styles without clobbering
the unspecified styles in the base theme.

Release Notes:

- Improved merging of `syntax` styles in the theme.