When you use custom templates for a Pages database, you'll often need custom CSS to go along with it to provide the styling. There's two main ways of doing this:
CSS files within Pages
Pages allows you to create CSS files, and then associate them with particular custom pages of your community (you create these in the AdminCP, under Pages > Templates > CSS). So simply create your CSS file, and associate it to the page that your database is displayed on.
The benefit of this method is it applies to all themes, so it's great if you want your database to look the same on all themes. Of course, this is also the drawback - you can't easily use it for per-theme customization.
Targeting the database classname in theme CSS
Alternatively, you can target the database classname in your normal theme CSS files. When a database is inserted into a page, IPS4 helpfully adds a classname to the body element, which makes it really simple to style that page in particular. If your database key is myDatabase, then the classname added to the body element would be cCmsDatabase_myDatabase. Use this in your selectors and you can style everything exactly how you need:
.cCmsDatabase_myDatabase .ipsButton_important { /* Style important buttons differently in this database, for example */ }
Combine both methods!
Of course, you can use both approaches when it makes sense. Create a CSS file within Pages for the basic structural styling that will apply regardless of which theme the user uses, and then in each theme target the database classname to customize it for that particular theme - perfect for the colors, font family and so on.