src/vs/platform/theme/common/themeService.ts/Themable/getColor
class Themable:	protected getColor(id: string, modify?: (color: Color, theme: IColorTheme) => Color): string | null {
		let color = this.theme.getColor(id);

		if (color && modify) {
			color = modify(color, this.theme);
		}

		return color ? color.toString() : null;
	}