No visibility (black background) with JupyterLab in Dark theme

[JonnyTran]

What happened:
With JupyterLab in Dark theme, the Bokeh dashboards doesn't have a white background so drawings are not visible.

What you expected to happen:
Bokeh dashboards should have a white plot background.

Environment:
JupyterLab 3.4.7
dask-labextension (pip install)

[ian-r-rose]

Thanks for the report @JonnyTran , there is some additional discussion here. I think the easiest fix would be to add a bit of explicit CSS to the background of the plot iframes. Would you be interested in submitting a PR?

[JonnyTran]

Sure! I'll take a look. If I have any questions I'll post it here or the orig issue.

[ian-r-rose]

Great! I think the easiest approach would be to create a new CSS class for the iframes here, and then to add that CSS class to the iframe widgets in the constructor here:

dask-labextension/src/dashboard.tsx

Lines 52 to 62 in 707ecb1

	constructor() {
	super({
	// Disable allow some iframe extensions to let server requests
	// and scripts to execute in the bokeh server context.
	// This is unsafe, but we presumably trust the code in the bokeh server.
	content: new IFrame({ sandbox: ['allow-scripts', 'allow-same-origin'] })
	});
	this._inactivePanel = Private.createInactivePanel();
	this.content.node.appendChild(this._inactivePanel);
	this.update();
	}

There are other widgets in the same file that add CSS classes (using addClass()) that you can look to for examples.