Improve Dark Mode support for shade tags and base colors

Summary:
Introduce dark colors for tags and shade tags to better integrate into the Dark Mode scheme. These are used for project tags, as a background color in project hovercards, as a background color when dragging a card on a workboard, and in other places.

These colors seem reasonably okay.

Also note that `lightyellowbackground`, `lightgreenbackground`, `lightvioletbackground` are currently not used in the codebase.

Refs T15056

Test Plan:
* Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Install http://phorge.localhost/applications/view/PhabricatorUIExamplesApplication/
* Look at the "Shades" section of http://phorge.localhost/uiexample/view/PHUITagExample/ (fixing Checkered with its background image is out of scope for this patch), hover over each tag to check that the border color is brighter than on non-hover
* Look at the colors of buttons in the "Simple Buttons" section of http://phorge.localhost/uiexample/view/PHUIButtonExample/
* Look at the tag colors on http://phorge.localhost/uiexample/view/PHUITypeaheadExample/
* For `{$lightredbackground}`, create a task with a restricted View Policy, add a comment `@username` with a user not allowed to see the task, look at the tag color and hover over the user to see their card - this one is not perfect but there's not much we can do given the red text color having to be standard red.
* Look at project tag colors on http://phorge.localhost/maniphest/query/all/
* Go to http://phorge.localhost/maniphest/task/edit/1/ and look at the colors in the "Tags" field
* Go to http://phorge.localhost/project/board/1/ and look at project tag colors
* Go to http://phorge.localhost/project/board/1/ and look at points in column headers
* Go to http://phorge.localhost/project/board/1/ and drag cards with different priorities, look at the card background color when dragging
* Go to http://phorge.localhost/T1 and look at project tag colors under "Tags"
* Go to http://phorge.localhost/T1 and hover over a tag under "Tags" and look at top part background color in the project hovercard
* Go to http://phorge.localhost/T1 and hover over a user with no verified email address under "Authored By" or "Subscribers" and look at the color of the "Email Not Verified" box
* Go to http://phorge.localhost/p/user/ and look at tags below the username such as "Administrator" or "Has MFA" or "Email Not Verified"
* Go to http://phorge.localhost/uiexample/view/PHUIObjectItemListExample/ and look colors under "Extra Things and Stuff" - this is not perfect but good enough for now
* Go to an EditEngine Form like http://phorge.localhost/transactions/editengine/projects.project/view/24/, select "Change Field Order", drag items and see better colors which allow you to read text

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15056

Differential Revision: https://we.phorge.it/D26370