Tools > Select UI Theme..

Select HaxeDevelop UI Theme

HaxeDevelop allows to change the look of the interface and the syntax. There are a few themes provided with the installation, but using the App Manager you can install even more.

Full themes
Changes the syntax and the UI. Provided in .fdz (installable zip) format

Syntax themes
Changes only the syntax. Provided in .fdz (installable zip) format

UI themes
Changes only the UI. Provided in .fdi (ini) format

Tip: You can find nice themes created by the community at the FlashDevelop sharing forum section.

Default HaxeDevelop Themes

Creating own themes

Syntax theme

You can use the Syntax Coloring panel.

If you are interested in the underlying XML files, find the documented xml-files here.

UI Theme

You can create a UI theme by creating a theme.fdi in the Settings\Themes folder of your user config files (top menu: Tools > User Config Files).

A excellent start would be this documented template file.