Missing icons and styling issues in version 4

## 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 https://github.com/jupyterlab/jupyterlab/issues/15629.