Accessing contents of a node id not working in .theme file
First, you are using the wrong hook. peeprocess_node won't send new variables into the page template. You need preprocess_page for that. The page template is not the same as the node template.

Second you are not assigning any new variables. You have to add or alter what is in $variables array or they are not passed to the template.

$variables['vanilla_spice'] = "testing the variable";