update notification for release 0.42

Any feedback? Thanks!
@peterflynn @pthiess @JeffryBooher @ryanstewart @redmunds @dangoor @RaymondLim @larz0 
How about:

"Brackets now has built-in support for editor themes, and includes default light and dark themes. Additional themes can be installed via Extension Manager."

I think that might flow a little better.