Change the default border color to a darker gray

[BreezeZin]

I find the default border color (gray 300) a little hard to see and suggest increasing it to gray 400.

https://imgur.com/4xaDkvp

The gray 400 color is almost identical to the border color for bootstrap.

I know that you can customize the border color in the config but I wanted to suggest the default be raised.

[wrabit]

Better would be to set the border as a variable so it can be set by user. I would like gray-200 for example, at the moment it seems it will require overwriting many places.