In reply to Coel Hellier:
> Hmm, so those of us with older themes should change to a newer one?
The recent index changes on Google are still in progress. They will affect the ranking of your site when
googling on a mobile device only, though it is expected that at some point in the future they will extend the ranking algorithms to cover other sized devices such as tablets.
If you want people on a wide range of devices to be able to easily use your website / blog then you need to ensure that it has some sort of responsive design.
It is important to go at this from an 'improving user experience' angle, rather than just worrying about what you have to do to please Google.
Luckily Google actually have some nice tools and advice to help people do this.
Start here
https://www.google.co.uk/webmasters/tools/mobile-friendly/