Use Sigil with the preview window open. Open a chapter where the text looks too scrunched in the code view. You should see some "class=" in the opening paragraph tag, eg. <p class="..."> right click on the class and select "go to link or style". That should open up your CSS to the correct style class.
There are two things you can look for that I noticed:
I would remove one or both of those and check the preview window to see if that is to your liking.
the negative value in letter-spacing causes the letters to be closer together than normal and the impact font causes a bold/scrunched appearance.