As per my understanding there is no way to hide the outline while hovering over any sheet. One quick question are you using floating worksheets? If yes then try to put those sheets in a horizontal container.
If you've put your worksheets in a container then check the properties like you have pinned any of the worksheet which causes one worksheet occupies the large real estate of your dashboard whereas other worksheets using less space.
However, if it doesn't help then it will be helpful if you share some sample mock up data in packaged workbook.