We haven't had any reports of similar issues regarding not able to access the Custom form styles feature in QBO. The steps should let you access it from your account. Let's try accessing your account using other supported browsers.
If it still doesn't work, I'd recommend reaching out to our Customer Care Support Team. They can further assist you with this as they have the necessary tools to take a look into your account and check on the root cause of the issue.
Here's how to contact them:
Click the Help icon at the top to open the QB Assistant window.
Tap the Talk to a human menu and enter the issue.
Next, click the I still need a human button and select the Contact us link.
For real-time assistance, choose the Start messaging or Get a Callback to speak with an agent.