It cannot be handled by StyleOne. The selector defines which content is be to be rendered, and the properties define the rendering to be done. Here the selector defines text within the H1-tag to be centered. Selectors have many faces. In StyleOne the selector part of a rule is defined under the key-tab.
More complete info is available from the CSS specification.
The semicolon is used to separate two properties within the same rule. In Style One you do not need to worry about these semantics, as the program handles this automatically for you. Cascading is the term used to define the behavior of style-sheet rules on each other depending on their origin, specificity and the order in which they are loaded. In general style-sheet rules inherit certain properties from their predecessors. It does however override the font-size property.