Roll20 is too bright.

If you create a new Map (without changing anything for grid or opacity) it looks like this: The only thing I changed was width and height. Can you see the grid on there? Happens for every campaign on every browser... To see the grid you need to change its color and opacity (and then it just looks ugly). In this state it is only useful for placing stuff on it but not to draw or anything alike.

You can also change the background color. I tend to make my default background color a dark blue, to spare my group's eyes when a page is loading. When you find a background and grid color combination that you like, you can go to Game Settings and choose that to be the default for new pages.

This I know... but it is annoying you have to do it yourself and if you create a new campaign and don't think of changing it in the settings outside you will be greeted with a blank page with nearly invisible grid... :D

You can set the default on your campaign settings page.

I go with a dark gray (almost all the way over to black) for my page backgrounds! The default white background is cripplingly bright for me, makes it inaccessible for my vision conditions.  Users have asked for years for Roll20 to tone-down the brightness but there has not been a change for that, yet. Fortunately, when it comes to the game tabletop & grid, the GM-creator can easily change the colors to whatever you want within the game table. It is not a bug, but it is a technical issue for accessiblity!  Sarah R. Cohen said: I tend to make my default background color a dark blue, to spare my group's eyes when a page is loading.

I use a black background since I like to keep margin all the way around the map. It helps cut down on display issues at some zoom levels, and when teamed with FoW gives me a handy place to store things the players can't see without going to the effort of Dynamic Lighting. For the grid, I usually use black, with a relatively low opacity, though this can change depending on the nature of the map. Because black-on-black is not very legible, I usually start with a white background and change it to black as the last step.

I too recommend the "Margins" technique as keithcurtis describes above, for the same reasons.  See less of the gray-hatch-pattern background in roll20 that sits behind the Page canvas.  Build your own dark-color safezones around all 4 edges of map. I do like dark-gray for the background color as for me it's a bit easier to differentiate from DL or FoW, to be sure where Fog Of War is revealed or remains hidden.