Revert "black background-color"

This reverts commit 97baa847d2aac8df98c71383242496972b4c9e15.

packages/presentation/src/components/Presentation.global.scss

@@ -1,4 +1,13 @@
-.reveal-viewport {
-  // adjust marp default theme
-  background-color: #000;
+:root[data-theme='light'] {
+  .reveal-viewport {
+    // adjust marp default theme
+    background-color: #fff;
+  }
+}
+
+:root[data-theme='dark'] {
+  .reveal-viewport {
+    // adjust marp default theme
+    background-color: #0d1117;
+  }
 }