docs(theming): update CSS properties to CSS custom properties (#3679)

Author: kgsatish

docs/theming/basics.md

@@ -31,7 +31,7 @@ Ionic has two **modes** that are used to customize the look of components based
 
 ## CSS Variables
 
-The Ionic Framework components are themed using <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_variables" target="_blank" rel="noopener noreferrer">CSS properties (variables)</a>. CSS variables add dynamic values to an otherwise static language. This is something that has traditionally required a CSS preprocessor like Sass. The look of an application can easily be changed by changing the value of any of the [CSS Variables](css-variables.md) Ionic Framework provides.
+The Ionic Framework components are themed using <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_variables" target="_blank" rel="noopener noreferrer">CSS custom properties (variables)</a>. CSS variables add dynamic values to an otherwise static language. This is something that has traditionally required a CSS preprocessor like Sass. The look of an application can easily be changed by changing the value of any of the [CSS Variables](css-variables.md) Ionic Framework provides.
 
 ## CSS Shadow Parts