Graham toshudhiman ok if you want to do it this way you can but you will have to do it for every page on your site the internal stylesheet option is really only for adding extra classes to a particular page not for the whole stylesheet
just be careful as there are a couple of strange bugs we have not nailed down yet

1/ the first class you define in a new internal stylesheet on the mobile templates will not have a .(dot) in front of the name and will not work so you will have to create a dummy (just add a name and leave the rest blank) you can delete this dummy class after you have created other classes

2/ do not create classes with names 2 characters long or your page will get an error and stop working as html and be displayed as text although editing the class name and re-saving will get rid of the bug

for new pages or when you make a change you will need to "copy stylesheet from other page" and select the page with your up to date stylesheet
"merge" replaces classes that are different in the target page with the ones from the donor page
"replace" replaces all the classes in the target page with the ones from the donor page

it is actually far less work to create the classes properly in global_stylesheet.css in the first place as these then automatically apply to all your pages and are available in the building tool blocks of all pages
You just need to learn how css classes work from a tutorial site