Is there any module that uses this bootstrap 3 core module?
Of course. You can check this one http://store.jahia.com/contents/modules-repository/org/jahia/modules/bootstrap3-components.html that offers a lot of great components.
Is there a way to customise my own CSS using fewer variables?
Yes. you can generate your own CSS file from the http://getbootstrap.com/customize/ and then put the generated CSS file directly in your template set.
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