rotating is possible, but only in 90° increments.
If you absolutely need a button looking like this: / then draw a small line at 45°, set its thickness to 20 points and define it as a button.
You can also draw a rectangle and set its fill to Image, and insert any image you like (but you'll have to prepare 3 of them, one for normal, one for hover and one for clicked).
You can also put any image, and put over it (or below it) a non colored button
png images are better if you want your button to change its fill during hover (while the button is below the image)