Migrate to bootstrap 4 in LeafWeb

WebCms/Content/bootstrap/_root.scss

@@ -0,0 +1,20 @@
+// Do not forget to update getting-started/theming.md!
+:root {
+  // Custom variable values only support SassScript inside `#{}`.
+  @each $color, $value in $colors {
+    --#{$color}: #{$value};
+  }
+
+  @each $color, $value in $theme-colors {
+    --#{$color}: #{$value};
+  }
+
+  @each $bp, $value in $grid-breakpoints {
+    --breakpoint-#{$bp}: #{$value};
+  }
+
+  // Use `inspect` for lists so that quoted items keep the quotes.
+  // See https://github.com/sass/sass/issues/2383#issuecomment-336349172
+  --font-family-sans-serif: #{inspect($font-family-sans-serif)};
+  --font-family-monospace: #{inspect($font-family-monospace)};
+}