Firstly, make sure you satisfy all requirements in the Before you install Question2Answer section in this link. Then, follow the steps in the Upgrading to the latest version of Question2Answer section in that link.
It is "safe". But again, you should backup all your data and files before upgrading to be 100% you have a plan B in case you face any issue that could be related to your particular configuration (PHP/Apache/MySQL).
Themes that come with the Q2A distribution (Snow/SnowFlat/Classic/Candy) are compatible with 1.7. Other themes might not. So you should ask the developer of the themes whether they're compatible or not.