@jiangcaiyang It works if you do it right. It doesn't take LESS variables, but it does work. Odds are the less styles have higher specificity, so your custom CSS rules are not applied. Use !important or something instead. It will certainly benefit you to not be held back due to incompatibility with newer versions of NodeBB in the future.
May I suggest that the INVITE function be available to at least admins permanently?
It's an incredibly useful and powerful feature to encourage registrations, case example, if you pick up a few emails while out networking or have a mail database you'd like to use to drive users to your forum.