Make border radius configurable

Carl Schwanrequested to merge
work/carl/configurable-border into master

Backport from the brise theme.

Only difference between the brise implementation is that the default is keep at 3. I'll be happy to change it to 5 but I'm pretty sure it will be harder to come to a consensus if we were to deviate the default value from the current value.

With 5 pixels radius:

image

@teams/vdg

Edited by Carl Schwan

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