error ” don’t use adjoing classes” and “Don’t use IDs in selectors”

Hi
1st of all – after a bit of a learning curve I’ve worked out how to use the CCS Editor – and begining to appreciate it.
Thanks

Now to my problem
I have a page that uses page builder embedded with
SiteOrigin Editor, Site Origin Image and Huge IT Video gallery on the “responsive theme” wordpress 4.4.2

I’m trying to set the background color of the page (the areas between the various widgets)
After playing around with the visual editor I got the CSS code below

Although this works, in the text box of the Custom CSS I get little yellow triangles, hovering over them gives me
“don’t use adjoing classes” and “Don’t use IDs in selectors”

What does this mean ?
Also, did I target the correct things – to change the background color do I really need to target all of the three things ?
Thanks
Yonatan

.pg-228-0 {
background-color: #b3ccff;
}

.grid.col-940 {
background-color: #b3ccff;
}

#wrapper.clearfix {
background-color: #b3ccff;
border-color: #b3ccff;
}