When making a copy from the default theme, include index.responsive.css

Signed-off-by: Suki <suki@missallsunday.com>
Check if responsive.css exists on the current theme and load the one from default if it doesn't

Signed-off-by: Suki <suki@missallsunday.com>
Merge pull request #2026 from MissAllSunday/stuff

When making a copy from the default theme, include index.responsive.css