I have just found what I need in the Customize button of the document well (to right of the Document Well, the drop down that lists the tabs that are not visible, at the bottom, Customize). This is available if you install the Productivity Power Tools 2013.
This opens the options for Productivity Power Tools. The relevant one is:
Custom Document Well -> Advanced. (Don't be fooled by the Color Coding option).
Ignore everything except the selected tab drop down (at the bottom of the dialog). Set that to VS Colors and I get a decent, readable selected tab, which was my main problem.
Can't imagine why this is so obfuscated, let alone why the default seems to render unreadable colors.