What is the best charging practice for my bike light's lithium ion battery?
You should fully charge the battery after every use.

Ignore outdated concerns about early NiCad batteries that exhibited a 'memory effect' when discharged repeatedly to the same Depth of Discharge (DOD)/ State of Charge (SOC).

Almost all secondary (rechargeable) batteries will benefit from regular, complete charging.  The less you discharge a battery, the more total lifetime capacity you will get from it.

That is, If you take the same battery and regularly use 25% of it's capacity then charge it, you will get C Wh of capacity out of it over its lifetime.  If you do the same, but use 95% of its capacity, you may find you only get 0.5C Wh over its lifetime.

Regarding Lithium batteries generally, it is extremely important to use the correct charger for the battery, or the correct mode on a multi-chemistry charger.  Otherwise there may be a fire...