themePath handling issue

[serhiisobol]

• themePath is platform-dependent (on Windows, the backslash is needed, on Linux/macOS forward slash)
• the error message when the file specified by themePath ("themePath is empty") is very misleading

On the left side of the screenshot inside package.json, themePath value is "style/index.css"
On the right themePath handling from commands.py

During the installation a jupyter lab extension with jupyter labextension link .
I am getting the path of styles with a backslash

[RRosio]

Hi @serhiisobol, thank you for submitting this issue! As this applies to JupyterLab, I have copied this issue over to the JupyterLab repository for further discussion there.

[serhiisobol]

Hi @serhiisobol, thank you for submitting this issue! As this applies to JupyterLab, I have copied this issue over to the JupyterLab repository for further discussion there.

Hello, thank you!