Hello,
Yes, you can edit from there, but we never recommend to do that. Because when you update the theme, it will loss your all changes. Instead, please add it from Additional CSS area by going to Dashboard > Appearance > Customize > Additional CSS.
Regarding your problem, it may need to give higher priority to your new code to override the theme style. Please try to add below CSS for color property;
body article.post-content {
color: #003366 !important;
}
Thanks.