You can try making changes on Appearance > Customize > Color > Content for article font colors.
For font size and family, you can go to Appearance > Customize > Typography > Headings.
Things that can’t be done within the Customizer UI is achieved through Custom CSS.
That said, can you link us to the site in question? In case we have to resort to custom CSS. We’ll need the link to inspect the site so we could take a closer look at its CSS selectors to assist you better.
A wise man once said:
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