improve line number background when set to transparent

[tgilgien]

A lot of themes set the line number background colour to transparent. The previous implementation would lead to the default widget background colour to be used.

This leads to poor results when using a dark editor theme with a light system theme or vice versa.

My proposal would be to use the editor background colour and draw a separator line when this is the case.

[Bzero]

Thanks for this PR, in principle I see the point of this proposal but I am not exactly sure if it works that well in practice.

If I understand right the PR addresses the cases where line_number_background_color = 'transparent' but the line_number_color is set according to the theme. However it seems to me that only very few pygments styles do this, most styles either specify both foreground and background color (in that case we do not have to worry about it) or do not set the line-number related properties at all and just inherit them from the default style (line_number_background_color = 'transparent', line_number_color = 'inherit'). In those cases the current implementation uses the default widget background colour with the default font color which seems reasonable to me. With this PR it seems that those cases become worse as it combines the system line number color with the editor background color which can be very similar when using a dark editor theme with a light system theme or vice versa:

Theme	line_number_color set	line_number_background_color set	master	PR
dracula	✔	✔
monokai	✘	✘

[tgilgien]

Right, I didn't notice that line_number_color is also often not set.
On my machine, it doesn't correctly inherit the system font colour when that is the case. I'll investigate this further.

Incidentally, the "native" theme does have a line_number_color set, but not line_number_background_color. However, I think that should be fixed in the upstream theme, it doesn't make sense to have only one of those set.

Thanks for the feedback and sorry for the false lead.