Refine selection of default themeing for highlighting

The setting for config option `aaa_bbb_{component}_focus_{tag}_bg` doesn't fall
back onto `aaa_bbb_{component}_focus_bg` because so far we only check if a
config entry for `aaa_bbb_{component}_focus_{tag}_{fg|bg}` exists. If *one* of
them exists (`fg OR bg`), we use that *combination*. If then the other entry
does not exist, it falls back to `'default'`.

This commit refines the fallback selection mechanism in that is tries
recursively to select themeing for the parent option (`aaa_bbb_ccc_fg` =>
`aaa_bbb_fg`) before finally falling back to the default setting.

Cf issue #25.
Closes #208.

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