Features

Roll Templates have some issues in dark mode - (Feb 2022)

Theme Switch

When a user switches to Dark Mode, the virtual tabletop adds the specific CSS class .sheet-darkmode to the <body> element. When flipping the switch back to light, that class is removed.

Theme Change

By adding color choices to the end of your sheets your sheet’s CSS, wrapped in a body.sheet-darkmode { } users will see the change when they flip the switch.

Updating Existing Sheet

Patching unsupported sheet
If you have a older sheet that's black & white and turn on dark mode, the background might turn dark and make some text unreadable. Adding the following to the sheet should fix it, in most cases:

.charsheet{
	background-color:white;
	color:black;
}

Adding darkmode support
This is what the Aborea sheet added to the CSS, to add dark mode:

body.sheet-darkmode button,
body.sheet-darkmode input,
body.sheet-darkmode optgroup,
body.sheet-darkmode select,
body.sheet-darkmode textarea,
body.sheet-darkmode .charsheet {
	color: #eee
}
body.sheet-darkmode .btn,
body.sheet-darkmode .btn.btn-default {
	color: #c8c3bc;
	text-shadow: rgba(24,26,27,.75) 0px 1px 1px;
	background-color: #1e2021;
	background-image: linear-gradient(#181a1b, #26292b);
	border-color: #3e4446 #3e4446 #43494c;
	box-shadow: rgba(24,26,27,.2) 0px 1px 0px inset,rgba(0,0,0,.05) 0px 1px 2px
}

It essentially only changed the default dark text color to a light color, and let the dark mode's default (dark) background make the sheet dark, while on the roll template it specified background-color and other things.

Overriding Dark Mode in Roll Templates
With how it's currently setup (3/8/2022), you should just need 3 classes (or equivalent) to override the dark mode styling. e.g. this:

<rolltemplate class="sheet-rolltemplate-template">
  <div class="template-wrapper">
    <!-- Template content here -->
  </div>
</rolltemplate>

.sheet-rolltemplate-template .sheet-template-wrapper .inlinerollresult{
  /*Your styling here*/
}

Sheet Examples

https://github.com/Roll20/roll20-character-sheets/pulls?q=is%3Apr+sort%3Aupdated-desc+dark+mode

Darkmode support

Sheets that updated to support the new dark mode

Pathfinder Community Dark Mode implemented
Aborea
Ironsworn - Starforged
Palladium Rifts by Grinning Gecko
D&D 5E by Roll20
^{^Character}_{_Sheet}
have implemented dark mode, but there is no public sourcecode for it.
Harnmaster3 Styling changes to make dark theme of sheet align better with roll20 dark mode. Update by sheet author: Now merged the previous light and dark themes with VTT dark mode. Turning on VTT dark mode switches the sheet into dark mode too. I took the liberty of moving my sheet up to the full support section.