ibdknox.css: fix tab coloring

In order to observe this change do following:
1. Set editor theme to "ibdknox".
2. In new-dark.css change var-highlight-fg to #0f0. Save new-dark.css.
3. Connect to 'Light Table UI'.
4. Open multiple editors, change content in one of them. Don't save!
5. Switch between editors, notice color changes in tab names
6. Undo/redo the change introduced by this commit. Goto step 5.
var-highlight-fg: #18B1CC;