To avoid making mistake, it’s better to ask that same person who wrote the dropcap and font styling to make additional changes to link colors.
If you decided to do it yourself, make sure you have the proper backup, so that you can go back to where it was if needed.
First, identify what Custom CSS plugin in use, and read its documentation, the setting page should be under Appearnce or Settings. I’m guessing it’s one from Jetpack plugin.
See Appearance > Edit CSS and check if the code in there is the same as this
https://paulwigmore.co.uk/monday/?custom-css
Put the additional CSS code under it.
The CSS for link color will affect all the links in the site, not just one page/post. Since there are different places the text links appear and parent theme might assign the color property separately, so we need to override/overwrite link colors in all the section.