The simplest way to change the background colour of an ApEx page is to edit thetemplate using the Application Builder.
Login to ApEx and navigate to your application. Click on 'Shared Components', and then 'Templates' to view the templates for the current application theme.
Navigate down to find the appropriate page template that you are using (tip: the 'References' column holds a count of the number of uses for a template, so if you're not sure look for something greater than zero). When you've found your template, click on it to edit the HTML elements of the template :
Here you can see we're using the 'style' tag to set the background colour (using background-color) to the hex colour #46C6BA. Standard HTML colour names such as 'white', 'blue' etc can also be used here.