@Adair Júnior, just to clarify - the custom UI themes mentioned by Boris are only usable for the newer responsive Dashboards.
If you’d still happen to use the older PixelPerfect dashboards (those where you can put images and layer the widgets on top of each other), I am afraid the custom themes are not applicable there.
And if it is the case, I am afraid that unlike the other visualizations, tables can not be made transparent in PixelPerfect dashboards.