Check out the pelican-bootstrap-3 help page on github.
What you can do is make a custom.css file that looks like this:
html {
font-size:150%
}
The trick is putting your custom.css file in your content folder and telling pelican where it is and then where it should be put in your output. You must also tell the pelican-bootstrap-3 theme where this file is and it will add it to the headers in your rendered output.
First, generally you would make a folder called extras in your content folder and put your custom.css file in there. This is where the path notation on the github page comes in:
extras/custom.css
You must now edit your pelicanconf.py
file to tell pelican where this file is located, and also to give this information to pelican-bootstrap-3. Add this to yours:
# tell pelican where your custom.css file is in your content folder
STATIC_PATHS = ['extras/custom.css']
# tell pelican where it should copy that file to in your output folder
EXTRA_PATH_METADATA = {
'extras/custom.css': {'path': 'static/custom.css'}
}
# tell the pelican-bootstrap-3 theme where to find the custom.css file in your output folder
CUSTOM_CSS = 'static/custom.css'