This is exactly what you can do, but this will impact many UI elements, and not just tabs in IMs.
Copy the
colors_base.xml file found in the
skins/default/ sub-directory of the installed viewer into a new
skins/custom/ sub-directory you will have to first create inside the
%AppData%/SecondLife/ (Windows) or
~/.secondlife/ (Linux) local settings directory.
Then edit this file to your heart's content, and finally start the viewer, select the "Custom" skin in the Preferences floater, Skins tab, close and restart the viewer to see the result (you will have to restart the viewer each time you modify the file to see any changes taken into account).
You may also copy some specific floater/panel definition XML files found in
skins/default/en-us/ of the installed viewer into the custom skin directory (i.e. in
%AppData%/skins/custom/en-us/ for Windows or
~/.secondlife/skins/custom/en-us/ for Linux), and edit them to change things (text, colors, layout, etc).
Be aware however that each time modifications will be done by me to the original skin files distributed with the viewer, you would have to change your own files to match (e.g. when I add/rename/remove colors to/in/from colors_base.xml or when I change the files for the floaters/panels you customized).
Also, be careful not to remove or miss-type elements/colors/values: bad XML definitions can cause viewer crashes.
