Grammarly doesn't work with Thrive Architect, but there is an alternative: ProWritingAid.
It's very similar to Grammarly. As you type, ProWritingAid underlines what is wrong.
It underlines mistakes in yellow, blue, or red.
And it does it while editing in a Thrive Architect text element.
Here's a screenshot:

ProWritingAid has a lot of other advanced features. Here are a few screenshots.

Click the image to enlarge

Click the image to enlarge
ProWritingAid works with a dark background color too.
Try it out.
You can try it for free with a limit of 500 words it can analyze.
You can upgrade with a 20% coupon off coupon. Use this link and the coupon HONEY20 at checkout. It's comparable to what Grammarly costs. It comes down to $5.27 per month for a 1-year license.
They have extensions for with most major browsers.