i fixed it by going to settings --- personalization --- themes and selecting a windows 10 theme it immediately changed from thick to normal. from there i could reselect my background and colour i wanted, hope that helps.

If coming here due to a blue box appearing around anything that you select (i.e. mouse click or tab to):

This is due to Narrator running.

To turn it off hold down the Caps Lock key and press the Esc key.

You can also permanently disable Narrator.