src/vs/workbench/services/decorations/browser/decorationsService.ts/DecorationRule/_appendForOne
class DecorationRule:	private _appendForOne(data: IDecorationData, element: HTMLStyleElement): void {
		const { color, letter } = data;
		// label
		createCSSRule(`.${this.itemColorClassName}`, `color: ${getColor(color)};`, element);
		if (ThemeIcon.isThemeIcon(letter)) {
			this._createIconCSSRule(letter, color, element);
		} else if (letter) {
			createCSSRule(`.${this.itemBadgeClassName}::after`, `content: "${letter}"; color: ${getColor(color)};`, element);
		}
	}