Sorry, but currently there's no way to change the background shadow for Dynamic Lighting or FOW. There's a thread here requesting the ability to customize the background color or replace it with an image. Right now, the black just represents zero vision, not foggy or unclear vision. For fog, what I've done is turned off ambient lighting in the page settings, then changed all the player's vision settings as shown in the screen shots below. This makes the token's vision begin dimming at 1 ft., and you can adjust the Night Vision Distance setting to suit for however dense you want the fog to be (I have it set here to 10 ft.) Since in my game actual Night Vision is not "infravision" or "heat vision", it's irrelevant in fog, so it allows me to substitute it for the token's normal vision in this specific case. Specifically, I use these settings for when the players enter an area affected by the D&D 5e wizards spell "Guards and Wards". You'll probably want to drag the player tokens from the journal and set their vision up ahead of time on the map that has the foggy conditions, and not copy them from whatever map they were previously on. If there are any token settings that don't automatically carry over, you'll need to set the tokens up before copying.
