You mentioned volts. There are two things that describe power, the voltage and the amperage. All USB chargers are "supposed" to supply 5v DC. I haven't seen a charger that delivers significantly more than 5v but I certainly haven't tested them all! Note that this is a NOMINAL 5v, it can vary some and will work fine with a well-designed device. Look on the charger - the one I'm looking at now says 5.2v. (in microscopic print!) You can easily verify the voltage for a specific charger with a multimeter and some tiny wires for probes. For the micro USB connector, measure between 5V and ground:
http://pinouts.ru/PortableDevices/mi...b_pinout.shtml
This says nothing about the current, the amperage. The USB spec requires power producing devices to supply at least 1/2 amp, 500 mA. Some can provide much more and some devices need more, specifically some Apple devices. I have some chargers with two ports, one for Apple devices that need more power, the second for other devices. Read all about the power that chargers can supply:
https://www.extremetech.com/computin...our-smartphone
About your lamp? It's impossible to know how they built it, but in general a device that needs a certain voltage, such as the 5v from a USB charger, will only pull as much current as it needs. (limited by resistance, impedance for AC) Even though a charger can deliver more current, it's up to the device using the power to pull what it needs. This is the same with the AC power in your house - if a device only needs a few milliamps that's all it will draw; if the device draws more the line will supply more up to the limit of the breaker.
As for your lamp, if it came with a charger, use it. If not and you are concerned, get a charger rated at 500 mA. The output voltage and amperage will be printed on the charger. The microscopic print on the one I'm looking at now lists the output at 1.35 amps - that's 1350 mA.
A well-designed device will not draw more current than it needs so it won't melt down and catch fire! However, battery charging circuits can be different - depending on the type of battery (Lithium Ion, Lithium polymer, NiCad, NiMH, etc) the battery can indeed be damaged by charging too long even if the voltage and current is exactly right. A well designed (sophisticated) charging circuit will manage the current but a cheap one may not. If the manufacturer doesn't provide charging time instructions either it doesn't matter or they don't care!
Is this a light you need to rely on? If so, I'd get one with replaceable batteries and carry a spare set.
Note that I don't claim to be an expert on this - this is just what I've learned from experience designing/building/repairing electronic circuits as a hobby, and from reading.
JKJ