The MSDN documentation on the WF designer shell has a very short section describing how WF designer fonts and colors work. Here is a bit more detail. You can manually change the colors and fonts used by the WF designer using the Tools->Options dialog box. The Workflow Designer for WF 4.0 does not have its own color scheme/font scheme context. Instead, it uses the environment fonts and colors. To change the color scheme:
Because the WF designer uses the environment fonts and colors, it automatically responds to changes to the Windows color scheme (from regular to high contrast mode and vice versa). Again, you have to start VS for changes to take effect in the designer, even though other parts of VS react immediately.
This is in contrast to WF 3.0 and 3.5 designer. It does not obey the environment font. Instead, in the Tools->Options dialog box, you click on the Workflow Designer node, and select a theme. This selection affects the color scheme for WF 3.0/3.5 but has no effect on WF 4.0. Also, WF 3.0/3.5 does not have built in high contrast mode support. You have to change the Workflow Designer theme to have the IDE environment match the OS.
Also, the hostable editor does not obey the environment font. The hostable editor obeys the font and color choices for the Text Editor. Follow steps 1 and 2 above, set the “Show settings for” to “Text Editor” and then party on the fonts and colors. These changes affect the big IDE as well as the little hostable editor IDE, so be careful. That means that if you customize the big IDE settings, you may want to customize the WF designer environment font as well so that your text stays readable in the WF designer.