OK I found another way to solve this…I put an image of a rectangle with round corners over a rectangle element. The image only has the corners in black and everything else is transparent. The rectangle is of my determined color. Works perfectly
EDIT: Should I be worried about performance when I have about 11×11 of those “constructions”? The images for the rectangle are in 256×256 for HD2, 128×128 for HD and 64×64 for SD.