Developers waste time installing themes that look good in demo screenshots but don't work well with their actual code. A preview system showing how themes handle different syntax would save this trial-and-error.
Build a web app that renders sample code in every theme with the user's preferred language. Include side-by-side comparisons and filtering by contrast/colorblind needs.
Monetize through premium theme sales or taking a cut of marketplace transactions. IDE vendors might pay for featured placement.
Start with a simple gallery of popular open-source themes with improved previews.
Biggest risk is competing with built-in theme stores in major editors.