Hello Harvliet Dalgety!

Let's review your questions:

1) The cover sections on the homepage and on the profile area have the own CSS classes: bx-cover-homepage and bx_persons_cover. So in your case, you may go to Studio->Lucid->Styles area, create the new Mix and then edit there Custom Styles field. Add there the own settings for the mentioned classes and press the "Save" button.

2) The same way to customize the necessary settings for the mobile devices, just use the media queries like this one:

@media (min-width:500px){/*Class name*/{/*properties*/}} 

3) Again, you may use the "Custom Styles" to set the specific color for the mobile menu.

4) About the "wrong crop" form your example I may suggest that it is just the cover view's settings. When you'd correct them like in the previous point the whole picture will appear.