Firstly, download it from here: https://build-system.fman.io/qt-designer-download Start Qt Designer.
Type label top-left, and drag the label over to the window. Then right-click, select Change stylesheet... and besides the Add Gradient button, click the down arrow and select background-color.
Hit New, see editor. You can click and drag the on the things blue arrows are pointing at to change the look. When done, hit Ok, Ok, get CSS code, copy it, and use it with the setLabelStyleSheet like so:
See result:
My example is quite ugly, but I'm sure others have better artistic skills than me (and more time to spend fiddling).