Changing the font size and color

Do you have a section in the dashboard named, “Custom CSS”, “Custom Styles” or “Jetpack”, or are you using a Child Theme?

I’m using jetpack

You don’t need a custom css plugin for this. Under the Styleheet css find this:
.entry-content, .archive-content {
padding: 12px 22px 0;
overflow: hidden;

Now add this beneath the last line of that (you can change the percentage to get different sizes):
font-size: 125%;

For such minor change I’d recommend using one of the plugins Andrew mentioned. Otherwise you’ll loose all changes once you update GamePress, and updates are coming ??

Good point alex27! In which case, you would add this to your Custom CSS plugin:

.entry-content, .archive-content {font-size: 125%;}