By default, Zabbix provides a number of predefined themes. You may follow the step-by-step procedure provided here in order to create your own. Feel free to share result of your work with Zabbix community if you created something nice.
To define your own theme you'll need to create a CSS file and save it as styles/themes/mytheme/main.css. You can either copy the files from a different theme and create your theme based on it or start from scratch. The rules in the main.css file will extend the ones that are defined in the base Zabbix CSS files located in the styles folder. Any theme-specific images must be placed in the styles/themes/mytheme/images folder.
Add your theme to the list of themes returned by the Z::getThemes() method. You can do this by overriding the ZBase::getThemes() method in the Z class. This can be done by adding the following code before the closing brace in include/classes/core/Z.php:
Note that the name you specify within the first pair of quotes must match the name of the directory under which the theme files have been saved.
To add multiple themes, just list them under the first theme, for example:
Note that every theme except the last one must have a trailing comma.
To change graph colours, entry must be added in the database table graph_theme.
Activate the new theme.
In Zabbix GUI, you may either set this theme to be the default one or change your theme in the user profile.
Enjoy the new look and feel!