The Style Builder dialog box (otherwise known as the style master) is deprecated. It still exists in the WebForms editor (in the "Format → New Style" section, but only after loading the design), but not in the new HTML and CSS editors.
The reason for this is that the dialog depends on the (deprecated) HTML constructor and because it is very outdated. Instead, you should consider writing styles in a CSS editor that has several advanced features (for example, CSS3 support, vendor prefixes, snippets, a color picker, and a number of other editing improvements).
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