fix(theme): fix text color defined in theme html, body

* instead of primary-color (=== blue), use foreground-1 (===dark grey) for the html, body theme text colo

src/core/style/themes.scss

@@ -2,7 +2,7 @@
 
 html, body {
   &.md-THEME_NAME-theme {
-    color: '{{primary-color}}';
+    color: '{{foreground-1}}';
     background-color: '{{background-50}}';
   }
 }