For any style to take affect it must be applied to a template. For information on how to apply a style to a Template see Templates Styles.
If it is a style that redefines an existing HTML element then you will see the effect immediately upon viewing the page in ProfitSuite.
If it is a custom style then it can be applied to any widget that uses style sheets. For information on how to apply styles please see the style sections of the Widget Guides that use them. For example the HTML Widget uses styles.