A theme styles pages. It can add content to the page header and footer and supply sundry files including a style sheet.
A theme uses a selectable template. A template changes the topology of a page whereas a style sheet changes only an element's style, eg, colour and scale. A template can when whether an element exists in a page and its order (relative to other elements); a style sheet can only change its appearance.
All pages include a default style sheet dictated by the theme's template. This gives the layout basic functionality, eg, making the menus pop up on mouse hover. If additional styling is need an additional style sheet can be uploaded to override and extend the values in the default style. All currently available templates include a default style sheet but this need not be so in future.
Creating and changing themes is not a WYSIWYG task. If you do not understand HTML and CSS leave it to someone else.