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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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (d07359c) to head (4f9d79d).
⚠️ Report is 18 commits behind head on master.
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Additional details and impacted files

@@      Coverage Diff      @@
##   master   #271   +/-   ##
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