src/vs/workbench/services/themes/common/colorThemeData.ts/TokenColorIndex/add
class TokenColorIndex:	public add(color: string | Color | undefined): number {
		color = normalizeColor(color);
		if (color === undefined) {
			return 0;
		}

		let value = this._color2id[color];
		if (value) {
			return value;
		}
		value = ++this._lastColorId;
		this._color2id[color] = value;
		this._id2color[value] = color;
		return value;
	}