Assuming it's using pure blacks for its dark mode and you're using an AMOLED display, it could save a bit of power, depending on how frequently you have that page open and how much screen space it's occupying. AMOLED displays illuminate each pixel individually, and pure blacks mean those pixels are just turned completely off, and aren't consuming any power.
The difference is pretty marginal though, to be perfectly honest. You might see some improvements to battery life on your phone using dark mode, but that's pretty negligible for most use-case scenarios.