Missing icons and styling issues in version 4

[jtpio]

Description

Icons are not properly displayed with the latest version of the extension.

There are also some styling issues (the version for JupyterLab 3 looks nicer).

Reproduce

Install jupyterlab-github and jupyterlab.

Previously with JupyterLab 3:

Now with JupyterLab 4:

Expected behavior

The extension should look the same as with JupyterLab 3, and icons should be correctly displayed.

Context

• Operating System and version: Linux Ubuntu 23.04
• Browser and version: Firefox 118 & Chromium 120
• JupyterLab version:

Noticed in jupyterlab/jupyterlab#15629.