Doc tweaks.

site/content/docs/4.3/getting-started/theming.md

@@ -215,9 +215,9 @@ You can also specify a base color with our color map functions:
 
 We use the `escape-svg` function to escape the `<`, `>` and `#` characters for SVG background images. These characters need to be escaped to properly render the background images in IE.
 
-## Add and Subtract function
+## Add and Subtract functions
 
-We use the `add` and `subtract` functions instead of the CSS `calc` function. The primary purpose of these functions is to avoid errors when "unitless" 0 is given to the `calc` expression.
+We use the `add` and `subtract` functions to wrap the CSS `calc` function. The primary purpose of these functions is to avoid errors when a "unitless" `0` value is passed into a `calc` expression. Expressions like `calc(10px - 0)` will return an error in all browsers, despite being mathematically correct.
 
 Example where the calc is valid: