Hi there, @situ. 

I can see how essential it is for you to make the font darker. Currently, the general feature to change the font color is unavailable in QuickBooks Online. In the meantime, it's best to relay your feedback to our Product Development team to improve your QuickBooks Online (QBO) experience. Here's how:

1. Go to the Gear icon and click Feedback.

2. Provide your feedback or product suggestions.

3. Click "Next" to submit.

To get updates about the status of your feedback, you may visit the Customer Feedback page.
 
You may also want to learn how to change the font color in your sales forms using the Custom form styles. This feature helps you change the font size and the form's color as well.

Refer to this article for more information: Customize invoices, estimates, and sales receipts in QuickBooks Online. 

If you need further assistance with changing font colors or anything else related to QuickBooks, please don't hesitate to reach out to us. We're always here to help. Have a nice day ahead!