Best Resource to override or customize the responsive design

My hero is scrambled in the built-in mobile version.  What is the best resource to learn how to override or customize the responsive design?

Sorry, I figured it out.  This is my first website on Squarespace.  I had made the assumption that since I couldn't edit content in mobile view that I also couldn't rearrange elements.  On my hero, the h1 goes to the bottom of the section content while h3 jumps to the top.  Also, on my content pages various full-width text boxes, buttons and images are completely out of order in mobile. Till now I've been developing from scratch and done customized responsive design for various viewports.  It's a little frustrating to not be able to change font sizes on one but not the other in Squarespace, and there doesn't seem to be a rhyme or reason as to how content is arranged by default in mobile.

5 hours ago, eforstmann said:

rhyme or reason

It's largely based on the order that blocks are added, but it also factors in page position to some extent, but this doesn't always work exactly how you'd expect!