At a very high level, if you want to mess around with things like padding and the OOB options aren't cutting it:

1. Deploy a custom skin (lots of steps around this; you can read up on it)
2. Use browser tools (F12) and figure out what CSS is controlling the padding of whatever thing you're trying to reduce padding for - then you can update it in your browser to get it looking how you want as a test. Once satisfied, make the changes to the CSS via your custom skin.

Alternatively you can overwrite CSS on your own on an ad-hoc basis, but this is hacky, so do it sparingly (if at all).

So to answer your question, yes, in general you can probably customize this sort of thing, but you need to have a good handle on how custom skins work in OBIEE and how CSS works.