Undo use of F5 which added a thick black line around fields and buttons

Solution 1:

You appear to have enabled the Accessibility shortcut.

To toggle it back off, option+command+F5

Solution 2:

As @bjbk indicates correctly, you have accidentally enabled accessibility tools, and need to de-activate them if they are unneeded or obstructive.

You can go up to menu bar at the top of the screen, to the Apple menu, click on the Apple icon at the extreme left of that menu bar, go to System Preferences, Accessibilty, and manually turn it off there, or as @bjbk indicated in their answer, you can use the Shortcut Key to toggle the state, which is now ON to OFF - click on option + command + F5 to accomplish this.