filediff: Use alpha value for chunk highlight from theme

This lets us customise the highlight for dark themes easily, as they
need significantly less highlighting for the chunk to stand out
properly.
linkmap: Use alpha value for chunk highlight from theme
bin/meld: Add support for a dark theme, following the GtkSetting

This commit just loads the dark CSS if it's present; there still isn't
any there. Since we're doing this, we also handle CSS files not being
found and complain appropriately.
Add preliminary dark theme

With thanks to Tobias Wolf for some colour suggestions.