Team hub — Forest after hours

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

radial-gradient(ellipse at 0% 0%, #294d45 0%, transparent 55%), radial-gradient(ellipse at 95% 95%, #253e48 0%, transparent 55%), linear-gradient(145deg, #152c28 0%, #101d20 55%, #0e171d 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.