Quick CSS clean up Dash Canvas toolbar

[jackparmer]

I love the utility of the Dash Canvas toolbar, but the icons look a bit oversized and uncentered to me:

I know plotly.js will likely fully replace dash-canvas in a few months (plotly/plotly.js#4374), so not worth spending too much time on this, but I think it could be improved with a few CSS overrides.

I adjusted the CSS in the console and came up with this, which I think is an improvement and matches the plotly.js modebar better: