Daily launchpad — Quiet aurora

After importing the board, open it, then Settings → Design → Background → Custom CSS.
Paste this background value and click Apply:

radial-gradient(ellipse at 10% 15%, #3b356b 0%, transparent 52%), radial-gradient(ellipse at 90% 95%, #24585c 0%, transparent 55%), radial-gradient(ellipse at 90% 0%, #294c6e 0%, transparent 45%), linear-gradient(140deg, #17223b 0%, #0e1929 100%)

This is the built-in background setting, not theme/component CSS.
Backgrounds are stored separately and are not included in the current board JSON import/export.