Adjust document tabs (v9.4)

[IanOfYork]

The background colour of the document tabs is the same as the background colour of the overall tab bar, which means that individual tabs are unclear.

Either
-- Add a single pixel contrasting border around the tabs.
-- Alter the tab background to be a slightly different colour for alternate tabs.
-- Alter the colour of the tab bar to be slightly different to the background colour of the tabs and add a pixel gap between the tabs.

Sync the changes with those of suggestion #13920