The in-game settings are accessible from the side menu (click the -icon on the upper left screen of your browser window to open the menu).
As a player, there are a couple of things you can configure.
When checked, this inverts the default snapping behaviour.
Because players can be using different screens (e.g. a projector, a laptop, a 4k tv), the default grid size of 1 grid cell == 50px might not be interesting. This setting allows players to modify the amount of pixels are used to render a single grid cell.
You can configure which colours you desire for your ruler, the grid and the fog of war. The opacity slider for the FOW colour is non functional, the DM can adjust this to their personal preference in the DM settings.
Last, but not least, you can select another language by choosing one of the supported languages in the drop-down menu. Note that PlanarAlly is developed in english and, thus, in case a translation of something in the user-interface is missing, the english version of the string will be displayed.