Menu styling confusion!

I have styled a menu for a test site using Megamenu. Its called mrbenmenu. I am using the Blocksy theme. I have set the location of the menu to be in the header all seems to be well until I go in to the customiser where the preferred menu i.e. mrbenmenu is replaced by the default styling from the theme.

Also the hamburger menu styling set up in Megamenu is not showing at all as it uses the default styling from the theme.

I have purged the cache every time before viewing the outcome of any changes but to no avail.

The attached images illustrate the issue:

https://mrbenretroclothing.com/mrbenonline/Screen%20Shot%202026-09-24%20at%2018.15.55.png

https://mrbenretroclothing.com/mrbenonline/Screen%20Shot%202026-09-24%20at%2018.16.25.png

What am I missing here? Any helpful comments are most welcome.

The page I need help with: [log in to see the link]