This looks good. Thanks for the contribution! I only have two suggestions:
1) there's a newer version of plugin_utils.py that fixes a few things. I suggest grabbing it and dropping it your plugin (
https://github.com/dougmassay/sigil-plugin-utils).
2) your gui will better match Sigil's darkmode theme if you add/change to the following (around line 180 in plugin.py:
Code:
icon = os.path.join(bk._w.plugin_dir, bk._w.plugin_name, 'plugin.svg')
# creating a python qt application
mdp = True if iswindows else False
app = PluginApplication(sys.argv, bk, app_icon=icon, match_dark_palette=mdp)