Ah yes, that is true... I forgot for a moment that the themes are not their own repository. In that case, your best bet would be to modify lavender by creating your own theme.
node_modules, run this:
$ git clone https://github.com/designcreateplay/nodebb-theme-lavender.git nodebb-theme-marcos2
$ cd nodebb-theme-marcos2
# .. make your changes here, update plugin.json, theme.json, and package.json and rename "lavender" to "marcos2"
From that directory (not the NodeBB main directory), you can use
git stash, etc.
git pull from that directory will pull in the latest lavender changes. Keep in mind this brings you out of the version release cycle for lavender, so caveat emptor!