Ensure text highlight color is preserved for reference file

In WebKit, the `::selection` pseudo-selector applies a ~0.5 opacity to the specified `background-color`. This means that in the HTML generated for for the "Keep current file as reference" command, the text highlight is far fainter than it should be. On my theme, it was almost invisible, and it made text search (with cmd-F) much harder because I couldn't see what was highlighted.

[The fix](http://stackoverflow.com/questions/7224445/css3-selection-behaves-differently-in-ff-chrome/7224621#7224621) is to specify the `background-color` as `rgba` with an opacity of `0.99`.