By background do you mean the level in which the character is standing (with the RV and gas can) or do you mean the transparent black background for the character stats?
If you mean the level with RV: that can be changed in EclipseStudio, or replaced with a seperate flash background.
If you mean the player stats background: i'm pretty sure its just a part of the flash, open the flash up with JPEXS decompiler and remove that part, it should be its own "sprite".
Open FrontEnd.swf (in JPEXS decompiler) -> Look for Shapes ->
Find "DefineShape (chid: 1539)" or "DefineShape (1539)"
Right-click either one and delete that shi.
Save your work and then rebuild "RSBuild.sln" and "RSUpdate.sln".
Note: no need to rebuild solutions if you are testing from studio
This should have the desired results.
Let me know how it goes.