The render_pixels button test rendered on a black canvas and asserted some
pixel differed from black. With no theme dir configured the active theme is
the embedded B/W fallback, whose Light mode defines accent (the button fill)
and text-primary as pure black on a white page — so a fallback button is
black-on-black and the assertion spuriously failed. Render on white instead:
the black fallback button stands out, and a real theme's saturated accent
also differs from white, keeping the structural check theme-independent.
