Refine background fallback values

[Tyriar]

Right now background fallback colors are very rough based on a quick run through of the Dark+ theme. Maybe we can improve upon it?

The fallbacks we land on may eventually be upstreamed to VS Code.

Related: #22

[Tyriar]

I went through the colors in microsoft/vscode#25327 (comment) and came up with the following notes.

{
	"colors": {

		"foreground": "", // A base color
			"editorForeground": "",
			"inputBoxForeground": "",
			"dropdownForeground": "",
			"listActiveSelectionForeground": "",
			"listFocusAndSelectionForeground": "",
			"peekViewResultsMatchForeground": "",
			"peekViewResultsSelectionForeground": "",
			"peekViewResultsFileForeground": "",
			"titleBarActiveForeground": "",
			"titleBarInactiveForeground": "",
			"activeTabActiveGroupForeground": "",
			"activeTabInactiveGroupForeground": "",
			"inactiveTabActiveGroupForeground": "",
			"inactiveTabInactiveGroupForeground": "",
			"activityBarForeground": "",
			"activityBadgeForeground": "",
			"panelActiveTitleForeground": "",
			"panelInactiveTitleForeground": "",
			"sideBarTitleForeground": "",
			"statusBarForeground": "",
			"notificationsForeground": "",
			"pickerGroupForeground": "",
			"editorCursor": "",
			"editorLineNumbers": "",
			"peekViewTitle": "",
			"peekViewTitleInfo": "",

		// color1 (primary accent color)
			"editorActiveLinkForeground": "",
			"focusedElementOutline": "",
			"highContrastBorder": "",
			"highContrastOutline": "",
			"inputBoxActiveOptionBorder": "",
			"buttonBackground": "", // Message box button
				"buttonHoverBackground": "", // Message box button
			"statusBarDebuggingBackground": "",
			"peekViewBorder": "", // Draw attention to the peek view with accent
			"peekViewResultsSelectionBackground": "",

		// background
			"editorBackground": "",
			"inputBoxBackground": "",
			"dropdownBackground": "",
			"editorWidgetBackground": "",
			"errorBackground": "",
			"infoBackground": "",
			"warningBackground": "",
			"errorBorder": "",
			"infoBorder": "",
			"warningBorder": "",
			"editorHoverBackground": "",
			"editorHoverBorder": "",
			"editorMarkerNavigationBackground": "", // F8
			"editorSuggestWidgetBackground": "", // Intellisense, how to set active item? https://github.com/Microsoft/vscode/issues/23506#issuecomment-297538193
			"editorSuggestWidgetBorder": "",
			"editorGroupBorder": "", // Thin border between editor groups
			"tabBorder": "", // 1px border between tabs
			"tabsContainerBackground": "",
			"activeTabBackground": "",
			"inactiveTabBackground": "",
			"activityBarBackground": "",
			"activityBadgeBackground": "",
			"panelBackground": "",
			"panelTopBorder": "",
			"sideBarBackground": "",
			"statusBarBackground": "",
			"statusBarNoFolderBackground": "",
			"debugToolBarBackground": "",
			"debugExceptionWidgetBackground": "",
			"debugExceptionWidgetBorder": "",
			"notificationsBackground": "",
			"peekViewResultsBackground": "",
			"peekViewEditorBackground": "",
			"peekViewTitleBackground": "",

		// Transparent white
			"listActiveSelectionBackground": "",
			"listDropBackground": "",
			"listFocusAndSelectionBackground": "",
			"listFocusBackground": "",
			"listHoverBackground": "",
			"listInactiveFocusBackground": "",
			"listInactiveSelectionBackground": "",
			"editorLineHighlight": "",
			"editorLineHighlightBorder": "",
			"editorRangeHighlight": "", // Highlight on rows on find
			"editorDragAndDropBackground": "", // DND overlay
			"activityBarDragAndDropBackground": "",
			"panelActiveTitleBorder": "",
			"sideBarSectionHeaderBackground": "", // eg. Debug collapsible sections
			"statusBarItemActiveBackground": "", // Mouse down
			"statusBarItemHoverBackground": "",
			"pickerGroupBorder": "",
		
		// Defaults
		"widgetShadow": "",
		"inputBoxBorder": "",
		"dropdownBorder": "",
		"buttonForeground": "", // How can we ensure contrast is sufficient?
		"scrollbarShadow": "",
		"scrollbarSliderActiveBackground": "",
		"scrollbarSliderBackground": "",
		"scrollbarSliderHoverBackground": "",
		"editorWhitespaces": "",
		"editorSelection": "",
		"editorIndentGuides": "",
		"editorMarkerNavigationError": "", // F8
		"editorMarkerNavigationWarning": "", // F8
		"editorFindMatch": "",
		"editorFindMatchHighlight": "",
		"editorFindRangeHighlight": "",
		"editorHoverHighlight": "",
		"editorInactiveSelection": "",
		"editorLinkForeground": "", // Do we need this? https://github.com/Microsoft/vscode/issues/25474
		"editorSelectionHighlight": "",
		"editorSuggestMatchHighlight": "", // Intellisen, not sure what this does https://github.com/Microsoft/vscode/issues/23506#issuecomment-297538193
		"editorWordHighlight": "",
		"editorWordHighlightStrong": "",
		"diffInserted": "",
		"diffInsertedOutline": "",
		"diffRemoved": "",
		"diffRemovedOutline": "",
		"titleBarActiveBackground": "", // macOS only
		"titleBarInactiveBackground": "", // macOS only
		"terminalAnsiBlack": "",
		"terminalAnsiBlue": "",
		"terminalAnsiBrightBlack": "",
		"terminalAnsiBrightBlue": "",
		"terminalAnsiBrightCyan": "",
		"terminalAnsiBrightGreen": "",
		"terminalAnsiBrightMagenta": "",
		"terminalAnsiBrightRed": "",
		"terminalAnsiBrightWhite": "",
		"terminalAnsiBrightYellow": "",
		"terminalAnsiCyan": "",
		"terminalAnsiGreen": "",
		"terminalAnsiMagenta": "",
		"terminalAnsiRed": "",
		"terminalAnsiWhite": "",
		"terminalAnsiYellow": "",
		"editorHeaderBackground": "", // Falls back nicely
		"editorSideBySideBorder": "", // BinaryResourceDiffEditor...?
		"editorGroupBackground": "", // Not sure when this applies
		"statusBarInfoItemBackground": "", // ???
		"statusBarInfoItemHoverBackground": "", // ???
		"peekViewEditorMatchHighlight": "",
		"peekViewResultsMatchHighlight": "",
	}
}

[Tyriar]

Discussion here microsoft/vscode#25482 (comment)

Another option is a hybrid of the multiple-level deriving:

background
  > selection, +x% light
    > sidebar selection background
    ...
  > strong background, +x% light
    > sidebar background
    ....

Minimum requirements is just background here, but it's recommended that you fill in all the "core colors" (selection, strong background, focus, etc.).

[Tyriar]

Some other thoughts I had on this was the idea of introducing "templates" which would need different sets of color, and color the different components differently (imagine the current model vs a flatter UI using the same color on sidebar, panel, editor, etc.