Themes are picky about trailing newlines

[dangoor]

During the 0.42 work on themes, we found that the Light theme was broken because there was no trailing newline. The problem may have been that the file had no contents at all (beyond comments), or it may have been the lack of trailing newline. If it's the trailing newline that's the problem, we should fix it so that it doesn't trip up theme authors.

[dangoor]

Fixed by #8594