First I will describe shortly that what is Theme? 🙂 All the website based on its theme which represents the design and layout, style and aesthetic to meet the different requirements/purpose. So we can say the theme is a entire multi page design which represent your Website looks.
There are two types of theme layout responsive and fixed. Responsive theme layout let your website correctly visible to all types of mobile devices such as smart phone, ipad etc. It will automatically break itself smoothly across multiple screen size/resolutions. In one word we can say flexible with all devices/monitors/screen.
There are another term called fluid. Fluid theme changes the width depending upon the screen/browser resolution. On the other hand fixed layout made for specific devices/monitor/screen/screen resolution.
So before purchasing a theme or design a theme keep in mind the above points.
Focus at your theme: Search engine optimization specially focuses on the theme which you are presenting through your website. A website is a complete introduction of your business or product, if you are successful to provide the information in a great way definitely, you will advertise your business or product in a better way. If you are not presenting your goal in a proper way and it is unable to grasp the attention of the reader; you can not gain anything from your web page. Search engine optimization tells you that you have to follow the tactics that may get the concentration of the internet user as much as possible. You have to have full grip on the topic otherwise you may loose your customers or clients.
Now a day all the themes are updated according to the Google guideline and policies but few of them are not. So be careful while selecting the theme for your website. Also look for the compatibility of the theme.