fix: apply theme style to embedded diagram svg (#7903)

* fix: Apply theme style to embedded diagram (#7903)

When diagrams are created with "adaptive colors" they automatically
change to light-/darkmode depending on the browser preference. This
leads to rendering problems when the settings in the browser differ
from the wiki.js theme setting.

Fixes #7677

* fix: Update color scheme for SVG in diagram class

Refactor SVG color scheme handling in diagram class.

* fix: remove duplicate svg first-child class

Removed unnecessary direction property from first SVG in diagram.

---------

Co-authored-by: Nicolas Giard <github@ngpixel.com>

client/themes/default/scss/app.scss

@@ -764,8 +764,17 @@
   .diagram {
     margin-top: 1rem;
     overflow: auto;
-    svg:first-child {
-      direction: ltr;
+
+    svg {
+      color-scheme: light;
+
+      &:first-child {
+        direction: ltr;
+      }
+
+      @at-root .theme--dark & {
+        color-scheme: dark !important;
+      }
     }
   }