Presets: Overriding
All loaded presets will be combined but you can override each file.
Take a look at docs/_merged_includes and override what you want to override by placing the same filename into _includes.
For example, to override the css files loaded in the <head>,
cp docs/_merged_includes/_joiningBlocks/head/40-stylesheets.njk \
docs/_includes/_joiningBlocks/head/40-stylesheets.njk
then edit the file to suit your needs.
Also works for _assets, _data ...
If you don't add .eleventyignore, you may receive error messages when running rocket build.