EDIT: Update, I have found a way to force night mode onto vortex (or any site) using the Google Chrome Extension "Dark Reader" I recommend looking it up and using it if you liked these ideas!
Two pretty simple Accessibility suggestions:
Toggleable on and off via Settings, removes the black overlay used behind the Pokéball loading animation when searching for Pokémon for epileptic users or those who want to remove the flickering when hunting
Toggleable on and off via Settings, changes the light palette of Vortex for a darker one (maybe even completely inverted) Or even just the white background for a black background for use at night or on bright screens for people with weaker eyesight. (Doesn't have to look pretty, just for ease of use)
Example 2 with inverse Colours: (very rushed, just an example)
Also things like Gender colours or Movement arrows clearly don't need to be swapped also.