Problem with CSS to change colors in JupyterLab notebook

I am not expert in CSS, so I am having some difficuties to understand the methods to customize the appearance of Jupyter notebooks. In particular, I would like to change the color of the math operators from purple to another color. If I correctly understand the mechanism, one should edit the index.css file under @jupiterlab\theme-dark extension. With the aid of Chrome Devtools and Firefox Developer Edition, I identified the line, it seems the following:
–jp-mirror-editor-operator-color: #bdcfde;

I tried to change the color code, but did not obtain any effect, also after reloading Jupyter. Could you help me to understand if there are other files or sections to modify in order to actually change the color? Thanks.