The symbolic attributes appearing in an Ipe document are translated to absolute values for rendering by a stylesheet that is attached to the document. Documents can have multiple "cascaded" stylesheets, the sheets form a stack, and symbols are looked up from top to bottom. At the bottom of any stylesheet cascade is always the minimal standard style sheet, which is built into Ipe.
When you create a new empty document, it automatically gets a copy of this standard style sheet (which does little more than define the "normal" attribute for each kind of attribute). In addition, Ipe inserts a predefined list of stylesheets. The list of these stylesheets can be customized using an ipelet, using Ipe's command line options, and an environment variable. By default, a new document gets the stylesheet basic that comes with Ipe.
The stylesheet dialog (in the Edit menu under Stylesheets) allows you to inspect the cascade of stylesheets associated with your document, to add and remove stylesheets, and to change their order. You can also save individual stylesheets.
The stylesheets of your document also determine the symbolic choices you have in the Ipe user interface. If you feel that Ipe does not offer you the right choice of colors, pen widths, etc., you are ready to make your own style sheet!