There are thousands of WordPress themes: Each tend to have its own way to change settings such as fonts. The controls may be under settings or under the Theme tab in the left sidebar on the dashboard. I like the Jupiter Theme among others and will use this as an example.
You need ‘Administrative’ user status to have access to Jupiter’s controls.
Click on Typography and General (until recently Global) Typography.
The default for body text is 13px and normal weight (400)
For H1 the default is 16px and Bold – (700). You can change these to your preference. Having made changes, remember to click on ‘Save Settings’.
For Font colours –
Stylng & Coloring / General Colors
There is a list of options including Body Text Colour
Open this menu and select your preferred colour.