Is it possible to add highlighting of multiple occurrences of selected characters in the notebook?
e.g. if I highlight a variable of xyz all occurrences of xyz are highlighted in yellow in the notebook.
You can accomplish this by using Chrome's Find feature:

There is also a Jupyter Notebook extension that does exactly this: https://github.com/jcb91/jupyter_highlight_selected_word
Now, to disable jupyter find feature to use Chrome find feature I found this thread useful:
https://superuser.com/questions/1407209/how-can-i-disable-searching-in-a-jupyterlab-cell