I hope the information below will help you to set a custom theme (for the ServiceNow Polaris UI) as default.
To realize this, a new record must be created in the User Preferences table (sys_user_preference):
1. Make sure the name is set to glide.ui.polaris.theme.variant.
2. The value should contain the the sys_id of the custom Theme UX style (table: sys_ux_style) which you want to set as default.
3. Make sure System is set to true.
Additional remark:
This default theme will not apply to users who explicitly set a particular theme. You can check (and remove if you want) the User Preferences table for any records with as name glide.ui.polaris.theme.variant and where the User is not empty.
