Neuroglancer Viewer has many accessibility issues

Upon running an Axe Devtools scan of our neuroglancer page, we surfaced a very large number of accessibility issues that seem inbuilt to the Neuroglancer library.

For example, class `.neuroglancer-side-panel-titlebar.neuroglancer-layer-side-panel-title` gets assigned `background-color: #393;` while the text on it in `.neuroglancer-layer-side-panel-name` gets assigned `color: white;` and this is not a high enough contrast ratio, resulting in this message: Element has insufficient color contrast of 3.65 (foreground color: #ffffff, background color: #339933, font size: 9.0pt (12px), font weight: bold). Expected contrast ratio of 4.5:1

Most other issues surfaced were either due to "Form elements must have labels" or "