Forum Moderators: not2easy
There's nothing wrong with having a site where some pages use one CSS file, some use another and some don't use CSS at all.
In the long run however, site maintenance is made considerably easier by using either a global or several sub-global CSS stylesheets... since this means you can modify the look of tens or hundreds of pages simply by editing a single file.
Once you get the hang of CSS on the new pages you're writing, you'll be able to apply it to your older pages which don't currently use CSS for styling and make those pages a lot more streamlined.