A common scenario of customization is when users first choose the theme for Redmine and then slightly modify it to fit their needs. As a result such users lose the possibility of upgrading the theme in future (as it now contains their changes).

Suppose we want to customize the Redmine Alternate theme. Instead of modifying the themes files under the public/themes/alternate directory, let's create a new theme based on this one. To do this let's create the packtpub subdirectory for our theme (this will be its name) in the public/themes directory. In the newly created directory create the stylesheets subdirectory and the application.css file in it.

Now add the following CSS code into this file:

@import url(../../../themes/alternate/stylesheets/application.css);

#top-menu { background-color: #040404; }
#header { background-color: #8db1c9; }
#main-menu li a { background-color: #404040; }

Here the first line loads CSS from the Alternate theme so we need to include only the difference into our customized one.