RDMO allows for a high level of customization by modifiing the Django templates as well as the static assets (CSS file, images, etc.). Django which RDMO is base on offers a powerful method for this. Inside your rdmo-app directory you can create a theme folder with a static and a templates directory inside:

mkdir theme
mkdir theme/static
mkdir theme/templates

Then add:

THEME_DIR = os.path.join(BASE_DIR, 'theme')

to your config/settings/

Templates and static files in the theme directory override files from RDMO as long as they have the same relative path, e.g. the file theme/templates/core/base_navigation.html overrides rdmo/core/templates/core/base_navigation.html.

Some files you might want to override are:

SASS variables
rdmo/core/static/core/css/variables.scss can be copied to theme/static/css/variables.scss and be used to customize colors.
Navigation bar
rdmo/core/templates/core/base_navigation.html can be copied to theme/templates/core/base_navigation.html and be used to customize the navbar.
Home page text
rdmo/core/templates/core/home_text_en.html and rdmo/core/templates/core/home_text_de.html can be copied to theme/templates/core/home_text_en.html and theme/templates/core/home_text_de.html and be used to customize text on the home page.

Note that updates to the RDMO package might render your theme incompatible to the RDMO code and cause errors. In this case the files in theme need to be adjusted to match their RDMO counterparts in functionality.