Support changing the border width of fcolorbox

[viocha]

Before requesting a feature, please search for existing issues.

Is your feature request related to a problem? Please describe.
The border width of fcolorbox cannot be customized

Describe the solution you'd like:
Support the usage of \setlength{\fboxrule}{0.8pt}

Link to or name of a (La)TeX package that provides the same feature:

Describe alternatives you've considered:

Additional context: