Wondering what is the best way to change the color of the banner at the top of all SharePoint pages.
Indeed using a Theme would be the easiest approach here. A very detailed blog is found here: http://www.iotap.com/sp2010/Blogs/tabid/470/EntryId/162/Applying-PowerPoint-2010-themes-to-Sharepoint-2010.aspx Also, the blogpost of Robin is a nice read also.
Well if you have a sigle master for all those pages, then adding a CSS link control in your master page and referencing a CSS deployed in the layouts can be a elegant solution.
I've just came across this today which might assist:
The first and easiest solution that comes to mind is to change your site theme, under site settings.