Use correct default color for new border config

Our default theme use default bootstrap variable and is not a real theme like other bootswatch themes. So be sure to use the default value when we do use variables before defaults bootstrap layer applies

This way we are explicit in the color we are mixing.

Related to #11828

Author: cderv

src/resources/formats/html/bootstrap/_bootstrap-variables.scss

@@ -272,9 +272,12 @@ $link-weight: $font-weight-base !default;
 $link-decoration: null !default;
 
 // border colors
+/// if a theme does not provide body-color or body-bg
+/// defaults to boostrap own default value for theses variables (in _variables.scss)
 $border-color: mix(
-  if(variable-exists(body-color), $body-color, #fff),
-  $body-contrast-bg,
-  30%
+  if(variable-exists(body-color), $body-color, $gray-900),
+  if(variable-exists(body-bg), $body-bg, $white),
+  15%
 ) !default;
+/// Make sure table border are the same as the border color (in case change in bootstrap default)
 $table-border-color: $border-color !default;