[DiapDealer]

There are no "base" Sigil styles. All styling is done via the epub itself, or the system rendering it.

As to the marked words; they are not links, they're anchors. That's why they don't take you anywhere. They're the targets for the links in the index. If Aldiko is underlining them, that's on Aldiko. You can try using css to eliminate the underline (the text-decoration property), but some reading systems automatically underline anything in an anchor tag--even if it's not a hyperlink. The epub (nor it's authoring system) can do anything about it if that's the case.