Added requirement @material/theme

Without this package I get the following error (after running npm prepare):
Error: Cannot find module '@material/dom/package.json'

Author: leonardas103

SVELTEKIT.md

@@ -10,6 +10,7 @@ npm install --save-dev @smui/card
 # etc...
 
 npm install --save-dev smui-theme
+npm install --save-dev @material/theme
 ```
 
 Create your theme files with `smui-theme`.