general question about templates/custom pages and styling

[user25148]

So i have a custom page (template?) created and the tabs all link to it fine etc...

Now usually with a html/php page I could write the code and then link it to a css stylesheet. Now I obviously want my custom page to be the same theme as the rest of my forum and be able to change themes with the rest of the forum. To do this, should I write all the html code for the page by editing the template in the adminCP? If so, how would I go about adding CSS to the page while keeping my theme? The additional.css file is not inherited across all my themes, so if I added it in there I would have to go through each theme and add the css to that themes additional.css file. Is there another way to do this?