fix: Skill icons not adjusting with system theme

[LachyTech]

This patch addresses an issue where the skill icons do not change according to the users' system theme preferences in Markdown output. If a skill is missing a separate dark icon, fall back to light icon.

Issue: https://github.com/danielcranney/profileme-dev/issues/171

Markdown preview:

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[danielcranney]

Thanks for the contribution @LachyTech and apologies for neglecting the project (we had a kid so life got busy!). All approved now!