Function assert_can_modify_theme 

pub async fn assert_can_modify_theme(
    db: &SqlitePool,
    current_user: Uuid,
    theme_id: Uuid,
) -> Result<ThemeRow, Error>

Permission gate for modifying (renaming / overwriting / deleting) a theme. Personal: must be the owner. Group: must be an owner of the group. Mirrors assert_can_delete_glw_data.

Errors

Returns Error::Forbidden if the user lacks write permission; Error::NotFound if the row does not exist.