1. Hold Shift key when opening Options/Preferences
2. Click Internal tab
3. Type "border" (without the quotes) into the Show only prefs containing: text box
4. Change Value from 3 to 2 and click Apply
5. If you like that thickness click OK. If not, change it to 1 and click Apply.
On my system 2 is best; 1 seems to be no border at all.
Quirk: I tried changing the color of the border too (just put "color" in the Show only box) and this worked, but was not saved when I closed MS13. So I got the yucky default border color on restart.
Oh well.