Toolbar icons disappeared

Hello everyone,
I'm sorry to ask this as I thought this was already an issue a few weeks ago. But I looked through all the issues and couldn't find it. So maybe I was wrong or I was not searching properly. Sorry for the inconvenience.

## Describe the issue
The toolbar icons have disappeared. This happened already a while ago after I reinstalled ipympl. So I thought it was just a bug in the version I updated to. But I updated ipympl and jupyter-matplotlib yesterday and still have no icons. Do you have any idea what I did wrong? The icons are the same as in the ipywidgets set, isn't it?

I have attached a screenshot at the bottom. All the buttons work just fine (also the tooltips), so it's just about the GUI.

Thanks for you help!

## Versions
Output of
```powershell
python -c "import sys; print('\n',sys.version); import ipympl; print('ipympl version:', ipympl.__version__)" ; jupyter --version ; jupyter nbextension list ; jupyter labextension list
```

```
3.6.10 (default, Mar 5 2020, 10:17:47) [MSC v.1900 64 bit (AMD64)]
ipympl version: 0.5.8
jupyter core : 4.6.3
jupyter-notebook : 6.1.4
qtconsole : not installed
ipython : 7.16.1
ipykernel : 5.3.4
jupyter client : 6.1.7
jupyter lab : 2.2.2
nbconvert : 6.0.7
ipywidgets : 7.5.1
nbformat : 5.0.7
traitlets : 4.3.3
Known nbextensions:
config dir: C:\Users\maxke\.jupyter\nbconfig
notebook section
jupyter-js-widgets/extension enabled
- Validating: ok
nbextensions_configurator/config_menu/main enabled
- Validating: problems found:
- require? X nbextensions_configurator/config_menu/main
contrib_nbextensions_help_item/main enabled
- Validating: ok
codefolding/main enabled
- Validating: ok
python-markdown/main enabled
- Validating: ok
hinterland/hinterland enabled
- Validating: ok
hide_input/main enabled
- Validating: ok
snippets/main enabled
- Validating: ok
code_prettify/2to3 disabled
tree section
nbextensions_configurator/tree_tab/main enabled
- Validating: problems found:
- require? X nbextensions_configurator/tree_tab/main
edit section
codefolding/edit enabled
- Validating: ok
config dir: C:\Users\maxke\.conda\envs\jlab\etc\jupyter\nbconfig
notebook section
jupyter-matplotlib/extension enabled
- Validating: ok
nbdime/index disabled
jupyter-js-widgets/extension enabled
- Validating: ok
JupyterLab v2.2.2
Known labextensions:
app dir: C:\Users\maxke\.conda\envs\jlab\share\jupyter\lab
@jupyter-widgets/jupyterlab-manager v2.0.0 enabled ok
@jupyterlab/debugger v0.2.0 enabled ok
@jupyterlab/toc v4.0.0 enabled ok
@krassowski/jupyterlab_go_to_definition v1.0.0 enabled ok
@lckr/jupyterlab_variableinspector v0.5.1 enabled ok
jupyter-matplotlib v0.7.4 enabled ok
nbdime-jupyterlab v2.0.0 enabled ok
spreadsheet-editor v0.4.0 enabled ok
```

## Reproduce
```py
plt.figure()
plt.plot(np.linspace(0,1, 100), np.random.rand(100))
```

Start by reproducing the issue with the provided plt.figure() and plt.plot() example in the listed JupyterLab 2.2.2 and ipympl 0.5.8 environment. Compare the toolbar rendering with the attached screenshot while checking the listed Jupyter extension configuration; done means the icons are visible while the existing buttons and tooltips still work.

Written by the indexing model from the issue text.