Themes: Error loading /config/configuration.yaml: mapping values are not allowed here

New to Home Assistant, so not fully aware of all aspects. My installation is via CasaOS running on Ubuntu server. My installation versions:

  • Core2024.5.5
  • Frontend20240501.1

So far everything is working like a charm, except I can not add any themes. I added in my configuration.yaml

frontend:
  themes: !include_dir_merge_named themes

I have tried adding in multiple themes, and each one gives me similar error:

Configuration errors
Error loading /config/configuration.yaml: mapping values are not allowed here
  in "/config/themes/visionos.yaml", line 224, column 75

I am unable it seems to install HACS due to my installation type. So my only recourse was to download the theme files to directory ‘themes’. I do not even know if I should continue working on this or if my installation will not support added themes (only been using HA for a week now), but it has been 3 days I have struggled with this issue and don’t know the next step.