Styling the body
You have already styled the div elements. You can style any part of a Web page. Edit default.css. Add a new blank line above the div line in default.css.
On the blank line add body then a space then an opening brace. Then add a couple of new lines and put a closing brace. The structure is the same as the div bit and is known as a rule set because styling rules are about to be added.
On the blank line between the braces put color:green; and then save it. View music.html in the browser. All text colour should change to green. If not check your spelling of color.
Size and borders
In default.css add a new line under the line which says background-color:yellow;. On that line add these:
width:250px;
height:250px;
border-style:solid;
border-width:10px;
border-color:blue;
Save and refresh the browser. The div elements should change. Look at these five new lines to work out why they now look like they do.
Index.html
Open this page in a browser to make sure that the styles work on index.html as well. They won't. Why? Fix that.
Try to work it out and fix it yourself but if you cannot the reason why the styles do not work on index.html can be seen if you hover here.