[FIX] web: provide border-color in primary_variables bundles

Prior to this commit it was not possible to detect/use the default
border-color in within the 'primary_variables` bundle.

This commit define a new `$o-border-color` variable to be used in BS
overrides and by components/functionalities that need it (eg. dark-mode)

task-3012970

X-original-commit: 29cd4ad3601cd6d3d86c8eaa747ddff600061c00
Part-of: odoo/odoo#103118

addons/web/static/src/scss/bootstrap_overridden.scss

@@ -98,6 +98,8 @@ $grid-gutter-width: $o-horizontal-padding * 2 !default;
 //
 // Define common padding and border radius sizes and more.
 
+$border-color: $o-border-color!default;
+
 $border-radius: $o-border-radius !default;
 $border-radius-sm: $o-border-radius-sm !default;
 $border-radius-lg: $o-border-radius-lg !default;

addons/web/static/src/scss/primary_variables.scss

@@ -89,6 +89,10 @@ $o-tooltip-text-color: $o-main-text-color !default;
 $o-tooltip-title-text-color: $o-main-headings-color !default;
 $o-tooltip-title-background-color: $o-view-background-color !default;
 
+// Components (BS)
+
+$o-border-color: $o-gray-300 !default;
+
 // Forms
 
 // o-inputs