Design studio — Warm paper

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

radial-gradient(ellipse at 12% 0%, #f9eee0 0%, transparent 55%), radial-gradient(ellipse at 100% 100%, #cfb8a0 0%, transparent 55%), linear-gradient(125deg, #eee7dc 0%, #e4dacd 55%, #ded2c1 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.