Step 3. Modify your website theme
In order for you to use Acquia Cohesion on your existing website, you need to add our cohesion-placeholder tag to your custom theme.
Acquia Cohesion uses render placeholders of inline CSS and as these cannot be naturally injected into the HTML template, the site's theme needs to contain <cohesion-placeholder></cohesion-placeholder> within the <head> tag.
To enable Acquia Cohesion on a custom theme you must add
cohesion: true to your themes .info.yml file.
After following the above steps and your theme is installed, a Cohesion rebuild is needed to create the required templates.