From the customize CSS dialog from the Global Preferences->Report Preferences->HTML you can choose a style that you want to be applied on the HTML reports. These styles are read from the AWR User Data/styles folder. There are several predefined styles (Classic, Business, Web and Modern) and you can also add your own styles by pressing the + button (Add Style button), or by manually adding the .css style in the styles folder. If you wish to add the style customstyle.css then you need to copy this file in the AWR User Data/styles/customstyle/ folder.

You can choose to have the selected style embedded in the HTML file, or to store it outside the generated HTML report. By default, the styles are embedded in the HTML report. Another way to link to a style is to include a style stored on a web server in your HTML report.