Improve visual appearance and usability of the source browser

[Jonathan Jacobs]

Some parts of the source browser are not particularly attractive (such as all the lines around line numbers) and attempting to select source code will include line numbers in the selection, it seems likely that the preferred behaviour would be to only select source code. This can be achieved with some CSS styles, see <http://stackoverflow.com/questions/826782/css-rule-to-disable-text-selection-highlighting>.