You can modify the CSS3 styles included in the QNX CAR platform by using Compass and Sass.
If you want to customize the CSS files on your system, you can start by inspecting the HTML source for each application so you can see each CSS file that is being loaded. The Web Inspector tool is ideal for this. For details, see the "Analyzing page resources" section in the "Debugging Web Apps" chapter of the HTML5 Developer's Guide, which is in the documentation set for the QNX SDK for Apps and Media.
You can directly access any CSS (.scss) files that reside on your target. For example, you'll find the base settings for the fonts and colors of the Midnight Blue HMI theme here:
/apps/common/themes/midnightblue/base.scss
gem install compass
The above command will install the Sass extension language as well.
compass version
compass init
In your working directory you should now see a file called config.rb, which you can use to configure the output for your project. For more information on configuring Compass, see the following:
http://compass-style.org/help/tutorials/configuration-reference/
The Compass site has a set of tutorials covering configuration, commands, best practices, and more:
http://compass-style.org/help/tutorials/
The Sass site includes a comprehensive set of reference documentation covering variables, mixins, and so on:
http://www.sass-lang.com/documentation/file.SASS_REFERENCE.html
The following post also has some useful information on Compass and Sass: