If I create a label and assign it a style class with a different font and THEN add style sheets using the Preview -> Scene Style Sheets menu, the font appears correctly. If I close the window and then reopen it, the font reverts to the default font. The style sheets are still shown in the Preview -> Scene Style Sheets -> Remote a Style Sheet menu. The CSS analyzer shows the correct font is being chosen. It just doesn't show the label in the right font.

If I remove the style sheets and re add them, the fonts then show correctly again. I am using two style sheets, one with a .root class to control all fonts in the app.