Hello Emilie, I am pleased to say that the latest version of the theme (version 1.1.10) now has an option to change the length (number of words) of blog posts excerpts.
Please update to version 1.1.10 and then navigate to ‘Customize’ > ‘Blog Options’ > ‘Excerpt Length’ to change this setting.
There is also an option to remove the excerpt completely if you wish to do so.