There are a couple different methods you could consider for creating sub-collections that will work in all of our themes.
1) Use the collection.sub-collections template.
2) You could 'tag' your products by different criteria. You can then enable the product tags filter in the Theme Settings > Collection Page section and when viewing this collection, all of the tags will be available to select.
3) If you use the sidebar on the collection page, you can display sub-links in the sidebar menu by using the same method described for the drop down menu. There are instructions for setting up your drop down menu here.
4) You could create an individual content page of collections. There are instructions for setting this up here