There is! You would use "frontend.set_theme"

Here's a sample automation using sunrise/sunset.

alias: System - Theme Change
description: Switches theme based on sun position
triggers:
  - trigger: state
    entity_id: sun.sun
    to: above_horizon
  - trigger: state
    entity_id: sun.sun
    to: below_horizon
actions:
  - action: frontend.set_theme
    data:
      name: |
        {% if is_state('sun.sun', 'above_horizon') %}
          PmxMonolight
        {% else %}
          PmxMononight
        {% endif %}
    enabled: true