The internal stylesheet editor is used to extend existing classes or to create new classes that will only apply on the current page
The CSS editor creates a
<style type="text/css">
tag in the
<head>
area of the page
The look will vary between templates but the basics are the same
Start by clicking on "add class" and give your class a name
Leave the pseudo class blank for now
[E + -] .myclass
Click on the name of your class then on add property
Click on Background and then on Background color
Use the menu to select one of the 16 preset color names or enter a 3 or 6 digit hex value
this is a basic css class and it will be available on the current page in the class selector for any building tool block that has a class selector or in HTML code blocks
For more complex classes and to learn about pseudo classes etc you should study a CSS tutorial
W3Schools CSS
You cannot style HTML tags directly using the editor