Like Led said, you can fiddle with the spacing and padding to adjust the pixel sharpness of the button skin.
Playing with a x1 multisample may help as well.
Also, a long time ago when I first started using those types of button skins instead of Delsim's work and the currently unavailable "NotCaith" skin, I found out a little trick that I still use with dominos.
When you unlock your actionbars, make sure they have a padding amount close to zero, it can be plus or minus a few.
Drag your action bar close to the edge of the screen but not all the way and when you let it go, it should snap to the edge of your screen.
When it snaps to the edge, it should remove the fuzzyness from the respective side of the button skin.
Then just move your actionbar back to where you want it to go.
Also, def make sure you run the Ui scale script or download the addon they linked above if you don't want to make a script macro.