Hi Gary,
Attached is a zip archive that includes a qt css style sheet that should lighten/brighten the look of the tabs in Sigil. It is just a text file and can be edited to change colours and things.
I found it online here:
http://doc.qt.io/archives/qt-4.8/sty...et-and-qtabbar
So please try grabbing the qt_styles.qss.zip attached here.
Then unzip it and make sure the file name becomes qt_styles.qss as this is the name of the style file that Sigil will look for. Rename it if need be. It is a text file and it can be looked at and edited by any text editor.
Now fire up Sigil and then go to Sigil->Preferences,
You will see a button at the bottom of the screen called "Open Preferences Location" and if you press it, a Finder window will open to where Sigil stores its Preferences, plugins, settings, and etc.
Simply drag and drop the qt_styles.qss into that new Finder window (ie. folder).
Now exit completely out of Sigil using the Sigil->Quit Sigil menu item.
Now restart Sigil and look at the tabs. They should appear brighter when "selected" and "not-selected" than they were before.
If this is not enough, we can play around with colours in that qt_styles.qss file with a text editor to make something even more readable for you.
Basically you are using Qt css rules to replace the Mac styles for your tabs just in Sigil.
Please let us know if this helps.
ps: You must be running a very recent version of Sigil for this to work. I recommend the latest version Sigil-0.9.9
Take care,
KevinH