how to create a background color for the text like in "start guide"

I think una is using a raw block with its own custom css in the Homepage.

Pages > System >Homepage

Add Blocks > raw > add

move the raw block to the left-bar under "Profile stats"

Open/Edit raw block and under "Content" box, add the una code and save

Clear cache (template/cache under Dashboard)

If you like, you can try their css/html attached here in the textfile or add your own.