Add --no-header flag and improve theme errors

[MaximIgitov]

Summary

• add a --no-header flag to suppress file headers
• emit a clear error when an unknown theme is requested
• add integration coverage for both behaviors

Test plan

• not run (Rust toolchain not installed on server)

[keith-hall]

Hi, do we need a new flag for not showing the header, considering we already have ways to remove style components?
And I thought we already have a fairly clear error when the theme can't be found? Please correct me :)