Don't include all themes from Shiki #7
Description
Apps using this lib appear to include lots of assets even though we didn't ask for them, e.g. https://github.com/rm-hull/calendar/actions/runs/19078057310/job/54498948056#step:8:12
see: https://chatgpt.com/c/690a45b7-bef4-8325-b843-f7dd0e22e9f5