Builder Style Manager: Scope and Limitations
The presence of Style Manager does not mean that one can escape writing custom CSS code. Style Manager can be used to set styles for some elements, it is not an extensive tool that covers each and every element on a webpage.
Style Manager & Custom CSS
In some cases, even when an option to style a particular element seems to be present in Style Manager, it might not actually work because the child theme's style.css overrides it. The "Cascading" part of Cascading Style Sheets (CSS) must be understood to avoid frustrations generally associated with Why the heck can't I style this with Style Manager?
The general answer to "What all can I style in Style Manager?" is: "Whatever you see and can style when you use Style Manager".
Now that you know Style Manager is not your swiss army knife for all your styling needs, what is? The answer is: A knowledge of CSS and Firebug. iThemes Builder customers can watch several videos on how to use Firebug to style various elements. We also compiled a list of CSS resources here.
It is not possible to apply one set of styles to the complete module or individual widgets of a module differently than other modules of that type. i.e., the styles that you set to widget bar modules will apply to all widget bar modules. Ex.: You will not be able to apply say, a blue background to just one widget bar module while the rest of them have been set to say, a red background via Style Manager. For such changes, style.css has to be edited manually.