themes: Define color for tree dnd borders

On social media (https://www.reddit.com/r/GIMP/comments/1hpybfx/minor_issues_with_300rc2/), a user pointed out that the green underline when dragging/dropping a layer was hard to see on the Grey theme.

Looking at it further, this was due to a system theme leak. In 2.10, the underline/overline color matches the foreground color of the current theme.

This patch defines that color to prevent the issue. It seems a straightforward change, but I'll tag @rangelovd and @brunolopesdsilv just in case there's some controversy I missed. 😄