We're the only theme with the appropriate heading svgs that make the headers and footers look as they should.
All the other themes fall back to Breeze, which is not a good look for some of them:
- the heading is too jarring compared to applet background
- the heading does not extend all the way vertically and horizontally
To solve this, to avoid degrading unmaintained themes and to give themes a chance to adjust on their own terms, this patch makes the heading visible only when the needed SVG exists in the theme.