No worries. Kris or someone else will eventually update the theme.
If you are curious on the technical side, this is related to Removing support for "template overrides" and mobile-specific templates.
If it’s only CSS changes, I would encourage you to create a new theme component:
Then, attach it to the official reddit-ish theme and move your CSS into this component.
This way, you get official updates while keeping your custom CSS.