Liferay's themes are designed in such way that they can be very easy to create. You can start by making changes in CSS files and as your customization requirements grow you can also make changes to the HTML that controls the page design.
Some of the technologies that you may need to know in order to make the best use of themes are:
CSS: If desired you can create a whole new theme just by changing a CSS file.
Velocity: a simple yet powerful tool to create templates. You will need to use it in order to customize the HTML generated by the theme.
XML: each theme has a configuration file written in XML. You will use this file to specify some settings of the theme.
To follow the examples of this guide you will also need some familiarity with using the command line. Alternatively you can use the Liferay IDE and use its menus instead of the commands used in the text.
But let's finish the introduction and get started with our first theme.