This is one of the bugs that I found while theming my MZ-Tools add-in with VS 2012 Dark / Light themes:
BUG: Visual Studio 2012 doesn’t theme multiline textbox with initial scrollbar on add-in toolwindow.
http://www.mztools.com/articles/2012/MZ2012028.aspx
And here it is the bug that I have filed through Microsoft Connect to be fixed:
Visual Studio 2012 doesn’t theme multiline textbox with initial scrollbar on add-in toolwindow
https://connect.microsoft.com/VisualStudio/feedback/details/766960/visual-studio-2012-doesnt-theme-multiline-textbox-with-initial-scrollbar-on-add-in-toolwindow#details
Fortunately I found an easy workaround that I have mentioned in the article.