This dialog box is displayed by menu item Options → Customize Configuration → Customize CSS Stylesheet. It allows customizing the CSS stylesheet being used by the active styled document view.

Figure 9.11. The "Customize CSS Stylesheet" dialog box

This dialog box lets the user type one or more CSS rules after the 3 first lines, which must not be removed or altered.

Note that a right click in the text area of this dialog box displays a useful popup menu.

Figure 9.12. Right-click popup menu of the "Customize CSS Stylesheet" dialog box

Clicking "Customize Stylesheet" checks the syntax of specified custom CSS stylesheet and immediately applies the changes to all styled document views using this stylesheet (called "Semantic" in the case of the above figure).

Clicking "Revert to Stock Stylesheet" lets the user revert to the stock CSS stylesheet (called "Semantic" found in stock "xhtml-config:css/xhtml5.css" in the case of the above figure).