The standard templates that came with the 2.7x branch should be already mobile responsive.
However, you should seriously think about an update. Your installation is more than 3 years without any updates, bug fixes and security fixes. Not a good idea with a software that can be used to potentially collect and store sensitive personal data. Your branch 2.7x is not supported anymore and you are not even on the latest version of this unsupported branch. Should be something around 2.7x I think.
I would highly recommend to update to the latest version of LS 3.x LTS (don't go with LS 4.x just yet, too many bugs still open).
I answer at the LimeSurvey forum in my spare time, I'm not a LimeSurvey GmbH employee.
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