Buggins - I see you're the developer. Do you have any intention of adding full css support? While using one unified style sheet for all epubs is not feasible given how different epub formatting is in different books. As an example, many Overdrive epubs don't even use <p> or <h1-6> tags - all formatting is buried in custom styles, and the books use <div> and <span> tags instead.)
Reading those books in an app without css support is an exercise in futility.
The newest version of Aldiko is bloated and slow, as the devs are more interested in drm support than a streamlined app. The market is ripe for a competing product to step in.