- Prepare for theme files, Put them in /themes folder as shown below I plan to use 2 themes: noctics and synthawave

2. Open configuration.yaml to edit frontend setting. We will use !include_dir_merge_named instead of !include then put the theme folder name themes then save the configuration.yaml
frontend:
themes: !include_dir_merge_named themes
3. Go to Home Assistant web interface > Configuration > Server Controls
- Press “Check configuration file” if it valid or not.
- Then go to “Server management” section and restart server.
- Wait for server restart for a while.
4. Look for your username at below left of screen then click to see your profile. You will have options to select theme there.

5. Once you install new theme. It’s no need to restart server – just do step 4. again.
Ref: https://www.home-assistant.io/integrations/frontend/#defining-themes