Clarify overrideThemeStyles function

[ribaricplusplus]

I was a bit confused as to what overrideThemeStyles function returns and had to look through the source code to figure it out. I added a brief clarification so that it is easier for the next person.

[codecov-io]

Codecov Report

Merging #271 (4f9d79d) into master (d07359c) will not change coverage. The diff coverage is n/a.

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