Below is the hierarchy of a default web project.

The css folder will contain 1 or more files each of these files should be considered a individual style. Within the style below (named current) is the master style sheet ( style.css) this style sheet controls your grids tables defaults etc.. in the sub directory named components you will find the folders and style sheets for your navigation and form components.

To copy or create a new style, create a new Grid component.

Select properties + the style name and then edit style.

Select (create new style) and choose to copy the current style or create a new one.
Name your style and save it in the css folder.

Once you have created your new style you can use the css editor to update it.