Yes. There is a dark-mode variables set for CSS. This can be found in this GitHub source code. You'll need to make sure that data-theme='dark' is set on the root HTML element for it to work.
The free DocSearch plan, doesn't include technical web support from the Support team. For further assistance, you can post your question on the Algolia Community Forum or Discord. For front-end issues, you can raise the issue directly on GitHub. In addition, you can review our DocSearch Documentation and FAQ.