DITA2Go generates CSS based on the formats to which you map the DITA elements in your document. To get the precise display you want, you might find that you need to edit the resulting style sheet in a text editor or a CSS editor. DITA2Go excludes most font tags, and optionally excludes typographic tags, because those can override CSS. What you get is very clean HTML, with @class attributes.
Although the default is to create a new CSS each time you run a conversion, there is a setting you can specify to retain the CSS as is; see §31.4 Specifying CSS file and link options. That is what we usually advise people to do if they customize the CSS. The downside is that if you define new formats (classes, in HTML) for your document, DITA2Go cannot add them to the CSS; you have to do that yourself.
DITA2Go gets the formatting information from the formats defined for your document via the [Templates]Formats file chain. DITA2Go provides reasonable starting defaults so you get good-looking results out of the box. You can change them to your taste in numerous ways; see §7. Configuring output formats.
By default, the first time you convert a document to HTML or XML, or generate HTML-based Help, DITA2Go creates a style sheet for the output. DITA2Go creates a new CSS file that contains all the paragraph and character format names from your document, based on the following:
§30.2.1 Assigning HTML tags and attributes to paragraph formats
Absent explicit CSS settings in [Classes], [ParaStyles], or [CharStyles], DITA2Go bases CSS class names on your format names, possibly reduced to fit CSS naming rules for class names; see §31.7.1 Understanding CSS class name restrictions.