Currently, the font colors on the templates can’t be modified (darkened, lightened or changed in color completely).
Unfortunately, if we change the color of the template for one user, it will change the overall look of the wedding websites for everyone else with the same design. Therefore it’s not an option for us to universally change the text colors at this time as some users may have purchased the invitations that exactly match their sites and colors.
However, you can use some HTML code to change the color of any typed text (with exception to the header). This help article explains this option in more detail.