Header title is barely readable with the default theme on a light theme #150

Since 2.0.0-beta.17 the default theme follows the user's theme. On a light theme the modal header turns white, but the file name in the header stays light grey (rgb(235, 235, 235)), so it's almost unreadable.

Steps to reproduce

1. Register a handler with theme: 'default'
2. Use a light Nextcloud theme
3. Open a file with that handler

Expected: the header title uses a dark text colour on the light header.

Actual: light grey text on white.

It looks like the header styles only cover the non-default themes. I switched files_mindmap to theme: 'dark' as a workaround for now: nextcloud/files_mindmap#375