Reactor includes many editable options grouped into five panels. Each panel has its options grouped too based on the section the options are related to.
In other words, you get Display, Skin Sections & Elements, Navigation, Custom Styles and Containers panels.
Display options let you toggle the visibility of just about every skin section and element on the page.
The Skin Sections & Elements panel includes options for customizing the appearance of different sections and elements on the page, like header, toolbar, content area, footer etc.
Custom Styles is an experimental panel that lets you add your own CSS styles to be included in the page skin to be generated.
The Containers panel includes options to create containers with different background colors, fonts, borders, etc.
More detailed information about the available options can be found in the documentation.