Is there a way not showing that 1px border on the right like v2.5 on the left?

If I am correct, to draw those borders, v3 doesn't display the exact pixel borders as they are in the border images, but for the left and right ones it only looks at the most left pixels, and for the top it calculates some average from that image.
Therefor you should be able to get the color you like by drawing the whole top border image in 1 color.