The themes/custom_vars.less template doesn't appear to exist.  From the 500 error page:

Django tried loading these templates, in this order:

    Using loader esp.utils.template.Loader:
    Using loader django.template.loaders.filesystem.Loader:
        /home/user/workspace/ESP-Website/esp/templates/themes/custom_vars.less (File does not exist)
    Using loader django.template.loaders.app_directories.Loader:
        /path/to/venv/site-packages/django/contrib/auth/templates/themes/custom_vars.less (File does not exist)
        /path/to/venv/site-packages/django/contrib/admin/templates/themes/custom_vars.less (File does not exist)
        /path/to/venv/site-packages/django/contrib/admindocs/templates/themes/custom_vars.less (File does not exist)
        /home/user/workspace/ESP-Website/esp/esp/program/modules/templates/themes/custom_vars.less (File does not exist)
        /path/to/venv/site-packages/django_extensions/templates/themes/custom_vars.less (File does not exist)
        /path/to/venv/site-packages/reversion/templates/themes/custom_vars.less (File does not exist)
        /path/to/venv/site-packages/form_utils/templates/themes/custom_vars.less (File does not exist)