This is what I did to change the theme of an existing Jekyll installation. Adjust these instructions to suit your needs.
Pull the new theme
We create a new orphan branch newtheme and ensure it's empty.
git checkout --orphan newtheme
git rm -rf .
git clean -dfx
Then we pull the theme files into it by adding the theme as an upstream remote. In this example I pull John Otander's Pixyll theme's master branch.
git remote add upstream https://github.com/johnotander/pixyll.git
git fetch upstream
git pull upstream master
Build the theme and test it.
bundler install
jekyll serve
Merge your changes
Now we merge our posts, configuration, etc. You can use Git checkout to copy a file or folder from your old Jekyll site. Note that this will overwrite the theme's file if it exists.
git checkout master -- _posts
Alternatively, you can copy a file under a new name, for example to merge it manually.
git show master:_config.yml > _config.yml.old
If you accidently overwrote a theme file, you can restore it.
git checkout upstream/master -- about.md
These are the files I had to copy, merge, adjust or remove:
- Markdown files in the root folder.
- Posts in the
_posts folder.
- Drafts in the
_drafts folder.
- The
_config.yml configuration file.
- The
Gemfile gem file.
- The
CNAME file (for GitHub pages).
- The
Rakefile (if any).
- The favicon files (if any).
- Manual theme changes such as Google Analytics, Disqus, SEO fields (if any).
Commit your changes, and don't forget to test the theme again.
Replace the master branch
Finally we replace our existing master branch with the new newtheme branch. Assuming we're on the newtheme branch:
git checkout newtheme
git merge -s ours master
git checkout master
git merge newtheme
Push the changes.
git push
And clean up the local newtheme branch.
git branch -d newtheme
That's it! You've successfully replaced your theme. If there's anything I missed, or you have anything to add, please leave a comment.
Updating the theme
If at any later point you want to update the theme to include the latest upstream changes, simply:
git pull upstream master
And fix any merge conflicts. Here I assume the upstream remote is still set to the theme's repository (you can check this with git remote -v).
