Add color-border variable

[skjnldsv]

After #6372 we have a variable that future apps may need (to avoid fallback 😄 )
I think this should be backported. What do you think? @MorrisJobke @karlitschek

[codecov]

Codecov Report

Merging #6649 into stable12 will increase coverage by <.01%.
The diff coverage is n/a.

@@              Coverage Diff               @@
##             stable12    #6649      +/-   ##
==============================================
+ Coverage       53.75%   53.76%   +<.01%     
  Complexity      22541    22541              
==============================================
  Files            1383     1383              
  Lines           86539    86539              
  Branches         1328     1328              
==============================================
+ Hits            46523    46528       +5     
+ Misses          40016    40011       -5

Impacted Files	Coverage Δ	Complexity Δ
core/js/js.js	61.83% <0%> (+0.55%)	0% <0%> (ø)	⬇️

[jancborchardt]

Looks good! Should we also directly add the border-radius variable for 3px?

[skjnldsv]

@jancborchardt Mind to do that on master and add the box shadow too? :)

[karlitschek]

backporting shouldn’t hurt I guess