Hi Anna,

if background color only in the logo column and you have predefined header then you have to use Custom CSS, here is example syntax:

#sp-logo {background: pink;}

But if you have Custom Header you can set background color in Layout > Header (Row Settings)

Example CSS to change whole header bg color:

#sp-header {background: yellow;}

Yes, in Helix documenation we have many tips: how to use Custom CSS or Customization tip as well.

Yes. Still this is possible and very easy.

Using Helix Ultimate Options and guide "Custom Header": https://www.joomshaper.com/documentation/helix-framework/custom-header

1. Disable Predefined Header
2. Then create your own custom header where you will use only "menu" module position in single column inside.
3. Then using Helix guide (again) you can center menu etc.

Many useful tips are here: https://www.joomshaper.com/documentation/helix-framework/customization-tips