Open Settings → Appearance. The Theme group controls the mode and the themes used for light and dark appearances. VS Code does not offer Yuri's theme-selection controls.
Choose what follows the system
Select a fixed light/dark mode or follow the system, then choose the theme for each mode. Check a conversation and a code block after changing it; both should remain easy to read.
Load a custom definition
Place the theme JSON in the connected host's Yuri data directory under themes/—normally ~/.config/yuri/themes/, or the directory selected by YURI_DATA_DIR. Then use Appearance → Theme → Reload themes and select the loaded entry.
On a phone, reloading still reads the host's directory, not files stored on the phone. The theme format reference describes the JSON fields.