Greetings Yousuf:
Probably the most efficient way to accomplish this – and learn from the process at the same time – is to look through the mega-menu CSS for JA Community Plus – most notably the background image configurations – and then “borrow” and/or outright copy-and-paste/overwrite the particular CSS lines and images from the JA Community Plus mega.css into your JA Mesolite mega.css.
As a JATC Member, you can download the JA Community Plus files toward this purpose – and then review the relevant css configurations (and related images) toward implementing them into your JA Mesolite based site.
Make sense?