fix(mdjs-preview): add css variable to border-color of viewer

Author: gerjanvangeest

.changeset/strange-houses-protect.md

@@ -0,0 +1,5 @@
+---
+'@mdjs/mdjs-preview': patch
+---
+
+Add css variable to style border-color of the mdjs-viewer

packages/mdjs-preview/src/MdJsPreview.js

@@ -686,11 +686,11 @@ export class MdJsPreview extends ScopedElementsMixin(LitElement) {
       }
 
       :host(:not([device-mode])) #wrapper {
-        border: 2px solid #4caf50;
+        border: 2px solid var(--primary-lines-color, #4caf50);
       }
 
       iframe {
-        border: 2px solid #4caf50;
+        border: 2px solid var(--primary-lines-color, #4caf50);
         background: #fff;
       }