The best way to do this is to put a value on the tenant.data object. From there you can access it in each theme.
So, for the tenant in the dev environment, set tenant.data.assethost to dev.example.com. For the prod environment, set tenant.data.assethost to dev.example.com.
Then, in your theme, you'd have something like this:
<link rel="stylesheet" href="${tenant.data.assethost}/styles.css"/>
The reason to use the tenant object is that it is available on every theme template.
This does mean that your tenant configuration will differ slightly between environments. You can also replicate this field value between tenants if you are using multiple tenants.