Hello,
Thank you very much for choosing our theme and for contacting us.
Each mega menu block is created by means of the HTML block, please check this manual to find how these HTML blocks are added to menu items: https://xtemos.com/docs-topic/create-mega-menu/
You need to detect which HTML block relates to this menu item, then find it in the Dashboard > HTML block and remove the background in the row or section. Find our documentation tutorial about this here: https://xtemos.com/docs-topic/html-blocks-usage/
If you have any questions please feel free to contact us.
Best Regards