Hello, Neighborhood.

I have just the steps to help you get the Custom Forms Style option back in your QuickBooks Online menu. 

It looks like you're using the new layout for the sales form in your QuickBooks Online company. The new layout doesn't use the styles from the Custom Form Styles menu, so this option is removed. 

To get the option back, you'll want to switch to the old layout. Here's how to do it: 

1. Click the + New button, then select Invoice.
2. On the invoice screen, click the Old layout button (located on the top right part of the page).
3. On the confirmation popup, click Leave. 
4. Proceed with the prompts to switch to the old layout. 

After switching, the Custom Forms Style option will reappear on the Gear icon menu. 

I'll also add this article in case you need help customizing your invoices: Customize invoices, estimates, and sales receipts in QuickBooks Online. 

In case you need to send statements to your customers for their outstanding balances, you can check this article for a guide: Create and send customer statements in QuickBooks Online. 

You're more than welcome to visit here again if you have other questions about customizing your sales forms. If you have concerns about your data or other entries, let me know in your reply and I'll help you out.