BuddyPress.org

The issue is that whatever theme you are working with has a style.css that applies rulesets using descendant selectors and on the elements you mention your style.css describes generic styles for things like ul li elements but as child elements of parent such as #content.

The stylesheet that appears to be created to apply the necessary styles required by buddypress i.e bp.css is styling on the class tokens.

As an example the div.item-list-tabs ul element requires and has a margin:0 applied in bp.css however in styles.css you have #content-body ul applying large margins and this ruleset has a higher specificity or weight due to the use of the ID selector which will override the clas selector described in bp.css.

I’m afraid that you have a little work to do to in getting whatever theme it is to work for BP, perhaps the fastest approach is open up bp.css and where you have :

div.item-list-tabs ul {}

Add to the beginning of the selector statement #content-body

#content-body div.item-list-tabs ul {}

This would enusure that the BP rules carry a higher weight and will win out or overrule.