You can change the plugin templates (old name: "internal pages"):

The variable "CUSTOM_INTERNAL_PAGE_DIR" defines the path you should put your own variant of plugin templates. The default is: "custom"

To change e.g. the main_menu plugin templates, copy all files:

from: .../pylucid_project/static/PyLucid/internal_page/main_menu
into: .../pylucid_project/static/PyLucid/custom/main_menu

PyLucid will first look into the custom dir and use these file. If file not found, PyLucid looks into the default directory.

Note: If you only will change the plugin template Stylesheet file, then you can better overwrite the CSS in your global Shylesheet.