Sidebar menu – won't be on a drop-down fashion

Hi everyone,

I installed Hueman theme on my website and fell in love with it.
Now, however, I’m having some difficulty customising here and there.
My main concern regards the sidebar menu, that can be easily set from the customise panel – it doesn’t behave like a normal menu, thus showing its subitems in a drop-down fashion, but it shows them all there.
How can I edit this? I couldn’t find the relative code anywhere. It’d be ideal if the subitems could stay nicely tucked away until users click on some “+”/”>” button to show the rest of the menu.

Also I’m wondering why it is so hard to customize the background. I’m trying to set an image background since my layout is boxed (using Full Screen BG Image) but apparently the rest of the website is too transparent so when the image is there content is impossible to read. There are just so many related codes in the style.css that I can’t find the right one.

Thank you so much for helping!

Link to website > https://www.inspiredbyplanetearth.net