Yes, that is because the theme you installed comes from a Git repository and we no longer allow eediting CSS/HTML on themes from git repos (or remote themes).
What you can do is create a new, empty theme component, add it to Graceful, and then make the CSS/HTML changes there.
Steps: in the theme overview click Install, then + Create New, give it a name then change type to Component, then click Create. In the next screen, add Graceful to the Include component on these themes setting, click and then the CSS you add to this new component will apply for users using the Graceful theme.