Make the playground header fit in with the rest of the site
Make the playground header fit in with the rest of the site

Fixes https://github.com/prettier/prettier/issues/3542.
existentialism/prettier,prettier/prettier,rattrayalex/prettier,rattrayalex/prettier,existentialism/prettier,rattrayalex/prettier,jlongster/prettier,existentialism/prettier,prettier/prettier,prettier/prettier,prettier/prettier,existentialism/prettier