Fix missing --theme-background-dark CSS variable for table borders

Author: Jani Giannoudis

docfx/templates/darkerfx/styles/main.scss

@@ -36,6 +36,7 @@ body.theme-dark {
     --theme-background-footer: #171717;
     --theme-background-table-alt: #212123;
     --theme-background-quote: #69696e;
+    --theme-background-dark: #555555;
     --theme-inline-code: inherit;
     --theme-inline-code-background: #454545;
     --theme-code-border: #404040;
@@ -62,6 +63,7 @@ body.theme-light {
     --theme-background-subnav: #f5f5f5;
     --theme-background-footer: #f8f8f8;
     --theme-background-table-alt: #f9f9f9;
+    --theme-background-dark: #cccccc;
     --theme-inline-code: #c7254e;
     --theme-inline-code-background: #f9f2f4;
     --theme-code-border: #e7e7e7;