How to customize mega menu in employee center portal?

[An Le]

Hi everyone, 

I am configuring the employee center now, I have a list of menu items which are catalog items and each item belongs to a category. 

Currently, my menu looks like this 

But I want the menu look like this. For example, Each item should belong to one category like Hardware, Software as image below.

What thing can I change?

Thank you so much.

[Susan Britt]

You can assign the catalog items to the appropriate Topic from the actual Topic record (Context Taxonomy > Topics).  On the Topic, from the "Connected Content" related list/tab at the bottom you can add the catalog item.  

[An Le · Accepted solution]

Thanks everyone, my problem was that I chose the wrong theme for the portal, thats why the megamenu did not show the expected menu items.

[SaurabhGidwani]

Can anyone let me know where can I change the CSS of the mega menu like the background color.