Hi KHDev,There is no restrictions on the Default template, do what ever you want. The Default template is not mobile or tablet friendly so keep that in mind for future updates and changes.On the other hand, you can find some responsive templates here that are mobile friendly http://www.osdateforum.com/responsive-template/ and work with osDate 2.65
To get started on making a new template in your template folder create a new folder and name it what you want your template name to be. Copy the index.tpl and index_header.tpl file from the defualt template into your new template folder. Now go into your admin panel and change the setting to your new template.Their you go you just started your new template. Now any file you want to modify copy that file from your default to your new template and modify it in your new template.Always leave the default template alone for it gives you a good backup to your tpl files if you make a mistake to a file in your new template.