1. The source plug-in does not support changing the font of index text: different plug-ins have different functions and setting options. If the source plug-in does not provide the option to change the font of index text, you may not be able to make changes through the plug-in.
2. The version of the source plug-in is older: some plug-ins may have added new functions or options in the updated version. If you are using an old version of the source code plug-in, you may lack the option to change the font of the index text. You can try to upgrade the plug-in to the latest version to see if it can solve the problem.
3. Other plug-ins or themes may interfere: Sometimes, other plug-ins or themes may interfere with the functions of plug-ins. You can try disabling other plug-ins or switching to the default theme to see if you can change the index text font normally.
If none of the above methods can solve your problem, you can try to contact the developer of the source plug-in or ask for help in the technical support forum.