There's an styletweak *almost* for that in "style tweaks -> pages and margins -> horizontal margins -> ignore horizontal paragraph margins"
which is just
Code:
p, li {
margin-left: 0 !important;
margin-right: 0 !important;
}
It should behave properly unless the rest of the book is coded by monkeys
As mergen3107 said you could also write your own styletweak.