How to make background color span full width

Site URL: https://seadragon-pear-6ndb.squarespace.com/

https://seadragon-pear-6ndb.squarespace.com/ password: op1

I would like the light green block to span the full width of the page and be only about as tall as the red box indicated in the screenshot. I also want to make sure there isn't excess white space below the box when I remove that bottom portion of the green box.

So far, here is the CSS I've tried. 

#yui_3_17_2_1_1582047611039_56 {
  padding: 20px 0px 0px 0px !important;
  background: #f1f5e6;
}

TIA for any help!

Use this code

div#tagline {
    background: green;
}