Here you can find list of different color schemes for syntax highlighting which can be used in HippoEDIT.
To install schema simply drop it in HippoEDIT data\color folder.
How to install extension with 1.50 and higher:
download bundle file (*.hebundle) and open it with HippoEDIT. Or use editor built-in on-line repository functionality.
How to install extension with 1.4x and below:
get extension using download link from the list, rename *.hebundle file to *.zip and unpack the archive into user data folder (current path can be found in About Box).