Welcome to Seven Forums Wiggy boy. It may be that you can change either of the settings Active window border or (more likely if I am interpreting your picture correctly) Border padding
Right click desktop> Personalize> Window Color at bottom> Advanced appearance settings> Item dropdown>
Note original settings should you want to set it back.
I think running the System File Checker will also set it back to original, but it will also reset any other theme modifications you have made. SFC /SCANNOW Command - System File Checker
Always best to make one change at a time, and see if you like the effect before making more.