docs: explain why the material theme files are basically empty

Author: hperrin

packages/site/src/theme/material-dark/_smui-theme.scss

@@ -3,6 +3,8 @@
   $background: #000000
 );
 
+// Basically just leave everything default (but with a dark bg).
+
 html,
 body {
   background-color: theme.$background;

packages/site/src/theme/material/_smui-theme.scss

@@ -1,5 +1,7 @@
 @use '@material/theme/index' as theme;
 
+// Basically just leave everything default.
+
 html,
 body {
   background-color: theme.$background;