I believe this is due to minor text rendering differences between the Designer and the actual TouchGFX application.
Within the Designer we use Windows font rendering (The Designer is built using C# and WPF) whereas the TouchGFX application uses FreeType behind the scenes.
We will fix this issue in a coming version of the Designer.
Can you verify that the problem is related to text - use a few boxes and/or images on top of your background?
Hope this helps.