You don’t really need to look at the users table here to display usernames. Data Explorer will automatically display avatars and username if the result of your query has a user_id column which the user_options table does have. 
Anyway, this topic got me personally interested so I spent a few minutes playing with data explorer and I got this little query which shows you user-selectable themes with the number of users using each theme:
WITH theme_stats AS (
SELECT theme_ids[1] AS theme_id, COUNT(*)
FROM user_options
WHERE array_length(theme_ids, 1) <> 0
GROUP BY theme_id
)
SELECT count AS "Number of users", themes.name AS "Theme name"
FROM theme_stats
JOIN themes
ON themes.id = theme_stats.theme_id
WHERE themes.user_selectable
ORDER BY "Number of users" DESC