The CB Login module uses module layouts. Those layouts are designed exactly after Joomlas. For Joomla 3.x that'd be the Bootstrap layout (matches protostar) and for Joomla 2.5 that'd be the Default layout. This means if your template styles Joomlas module it should also style CBs. If it doesn't then it's not restyling it strictly with CSS and is instead using a custom layout, which won't affect CBs login module. To restyle elements like the button you can use CSS or create a module layout override. There is no CSS file specific to CB login module as again it's styled by Joomla.

Our login module also supports adding custom CSS classes to various elements quickly and easily within the modules parameters under the Style Options tab.

CB 2.1 is also adjusting how we take over Joomlas user component and will handle login post. This means you'll be able to use whatever login module you want and it'll still trigger a login through CB.