You should be able to use E-junkie with any WP theme you choose, as we do not provide or impose any particular page template or storefront CMS you'd have to use; how you design/generate your pages and product listings would be left entirely up to you.
Once you set up your products in E-junkie Seller Admin, we provide you with ready-made HTML codes for your cart buttons. You would simply copy the codes from us and paste them into the HTML source of your own Web site pages, wherever you want our buttons to appear among your own layout, text and images. Our standard cart would appear as an overlay "inside" your own page, fitting within your site's existing layout and branding.
For WordPress sites, we do at least recommend using the Raw HTML WP plugin to help ensure that the WP software won't mangle our button codes after you paste them in; other tips you may find helpful are provided here: