Hamburger button for products page in mobile page display

I'm sorry, I noticed on the Droopler Demo that the button isn't actually white, like in my setup. And better yet, the button has an icon that is different from the hamburger button at the top of the page, in order not to confuse both menus. My apologies for the wrong heading, and... Great Job!

Then my question should have been: Why is the filter menu icon not displaying in my setup? I bet the quick answer is: because my setup is non-ddev. Okay. Ddev is to be preferred because is has all the right software and configurations for development. But it should also be usable on a real live server, right? So what server software or configuration am I missing here? It's hard for me to make an educated guess, because I'm still trying to educate myself. So please someone fill me in here? Thanks!

I installed Droopler with DDev, just to find out if this issue is also visible in DDev. And to my surprise, in DDev, Droopler mobile screen also displays a white button instead of displaying the 'filter' icon. So I remain curious what's causing this, and how to fix it?