I have started to create a website for my business, and it was all going splendidly until I wanted to change the background colour in auto nav. It does not work by clicking on "design" and then "background." From searching the forum I understand this has something to do with the specific theme I have chosen (Greek Yogurt), and that I have to input something in CSS. I have never used CSS before and this is where I am completely lost. Is someone willing to give me step by step instructions that a complete noob can understand?

I think i will need a specific code for the background colour I will want - how do I find this code? Thanks in advance!