Specify Minimum Width

Hi, on my page I have a webform. Looking at it on a computer screen it is fine. However when you view it on a phone it becomes a mess (because the webform is somehow incapable of being responsive. Showing the form at its full size causes the theme to only show 70% of the page, making it way too thin.

If you have a look at my page and then resize your browser window to about the width of a phone it is really obvious (in case I am not explaining it right).
<a href=”https://www.lifeimproving.com/unlock_your_brains_limitless_power
“>https://www.lifeimproving.com/unlock_your_brains_limitless_power

I think the solution would be for me to set a minimum width for the whole theme. (I notice that there must be an existing minimum width specified as I can see that you cannot shrink it down to nothing.

Where in the code can I find this?

Thanks.