Your theme isn’t aware of BP components so can’t add any particular styling to them other than whatever default properties it styles such as links and it’s major structural elements, by the same token BP cannot be aware of the variances of theme it may be slotted into so has to keep it’s styles to an absolute minimum to enable a basic layout for components, thus the onus is on the user to then take and extend the BP styles to better fit to their requirements; for this you will need a basic level of CSS knowledge.
