Not sure if I'm using the right keywords trying to find an answer, but I'm building a theme from scratch for the first time and found that when I mouse over a block, or add a new block area the highlighted/selectable area (that I need to click on to add a block, or edit) is offset down horizontally.

When I switch back to a green salad theme the issue goes away, so I'm clearly doing something wrong, or not adding something.

If you look at the attached highlight-bug.jpg you'll see what I'm trying to describe for an existing content block. hightlight-bug-2.jpg shows how far offset it is with an area that has no block assigned yet.