There is no free bootstrap theme available. But Dude Theme is LESS BOOTSTRAP and looking nice but its not free and doesn't know if it is compatiable to Q2A version: 1.6.3
Otherwise you can integrate bootstrap theme on q2a platform.Now I have created my own bootstrap theme with new layout
Cleanstrap: free, open source and responsive.
Uses bootstrap (proof: https://github.com/Towhidn/CleanStrap/blob/master/cleanstrap/js/bootstrap.js)
Compatible with 1.6.3, I used it for a few months on 1.6 no problems.
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April 20, 2021: Q2A 1.8.6 released!