Issue-3895: Make variables root configurable

sass/base/generic.scss

@@ -32,7 +32,7 @@ $pre-font-size: 0.875em !default;
 $pre-padding: 1.25rem 1.5rem !default;
 $pre-code-font-size: 1em !default;
 
-:root {
+#{cv.$variables-host} {
   @include cv.register-vars(
     (
       "body-background-color": #{$body-background-color},

sass/base/skeleton.scss

@@ -9,7 +9,7 @@ $skeleton-block-min-height: 4.5em !default;
 $skeleton-lines-gap: 0.75em !default;
 $skeleton-line-height: 0.75em !default;
 
-:root {
+#{cv.$variables-host} {
   @include cv.register-vars(
     (
       "skeleton-background": #{$skeleton-background},

sass/components/navbar.scss

@@ -101,7 +101,7 @@ $navbar-colors: dv.$colors !default;
   z-index: cv.getVar("navbar-fixed-z");
 }
 
-:root {
+#{cv.$variables-host} {
   @include cv.register-vars(
     (
       "navbar-height": #{$navbar-height},

sass/form/tools.scss

@@ -15,7 +15,7 @@ $label-colors: shared.$form-colors !default;
 
 $field-block-spacing: 0.75rem !default;
 
-:root {
+#{cv.$variables-host} {
   @include cv.register-vars(
     (
       "label-color": #{$label-color},

sass/grid/columns.scss

@@ -7,7 +7,7 @@
 
 $column-gap: 0.75rem !default;
 
-:root {
+#{cv.$variables-host} {
   @include cv.register-vars(
     (
       "column-gap": #{$column-gap},

sass/themes/_index.scss

@@ -7,7 +7,7 @@
 @use "dark";
 @use "setup";
 
-:root {
+#{cv.$variables-host} {
   @include light.light-theme;
   @include setup.setup-theme;
 }

sass/utilities/controls.scss

@@ -16,7 +16,10 @@ $control-padding-horizontal: calc(0.75em - #{$control-border-width}) !default;
 
 $control-focus-shadow-l: 50% !default;
 
-:root {
+@debug cv.$variables-host;
+@debug iv.$radius;
+
+#{cv.$variables-host} {
   @include cv.register-vars(
     (
       "control-radius": #{$control-radius},

sass/utilities/css-variables.scss

@@ -6,6 +6,8 @@
 @use "initial-variables" as iv;
 @use "functions" as fn;
 
+$variables-host: iv.$variables-host;
+
 @function buildVarName($name, $prefix: "", $suffix: "") {
   @return "--#{iv.$cssvars-prefix}#{$prefix}#{$name}#{$suffix}";
 }
@@ -419,7 +421,7 @@
 
 @mixin system-theme($name) {
   @media (prefers-color-scheme: #{$name}) {
-    :root {
+    #{$variables-host} {
       @content;
     }
   }

sass/utilities/initial-variables.scss

@@ -153,3 +153,4 @@ $class-prefix: "" !default;
 $cssvars-prefix: "bulma-" !default;
 $helpers-prefix: "is-" !default;
 $helpers-has-prefix: "has-" !default;
+$variables-host: ":root" !default;