Is Wooxy legal?

Submit a ticket to riot support asking if it is legal:

https://support.riotgames.com/hc/en-us/requests/new?ticket_form_id=38204

Seconded. Submit a ticket and check.

That said, the usual guidelines are:

1. Anything not from Riot is use at your own risk. Riot is not at fault for anything that goes wrong.

2. Anything not from Riot could be bannable at any time - Riot can change their mind if something is allowed or not any any time.

3. Generally, if something only modifies what you see and and does not present more information than you would not normally have in the official client, then you should be fine. For example, custom skins, thematic maps, etc.

4. If an application/modification/whatever gives you a competitive advantage over somebody else, then it's prohibited and you will likely be banned. For example, custom themes that show teammate cooldowns (other than the ult) or any enemy cooldowns, timers you normally wouldn't have (which is only inhibitors and "minor" jungle camps and scuttle now AFAIK), scripts, etc. Just stick to skinning mods to be "safe".