refactor(lib): remove unused theme keys

[hi-ogawa]

• closes chore: slim-down css variables in preflight css #83

By filtering useless variables, it saves about 10KB:

# before
$ pnpm -C packages/app -s build
vite v4.4.9 building for production...
✓ 130 modules transformed.
dist/index.html                   2.20 kB │ gzip:   0.96 kB
dist/assets/index-cd362aed.css   52.80 kB │ gzip:  10.06 kB
dist/assets/index-0022a56b.js   389.10 kB │ gzip: 122.90 kB │ map: 1,532.15 kB
✓ built in 2.16s

# after
$ pnpm -C packages/app -s build
vite v4.4.9 building for production...
✓ 130 modules transformed.
dist/index.html                   2.20 kB │ gzip:   0.96 kB
dist/assets/index-4bbc95c7.css   42.30 kB │ gzip:   7.67 kB
dist/assets/index-a845fe92.js   376.89 kB │ gzip: 119.86 kB │ map: 1,506.09 kB
✓ built in 2.15s

generated css