There's a quiet frustration in the post from u/Voooow, and we understand it completely. You upgraded your Excel, and suddenly the interface feels like it belongs to someone else, green ribbon, green accents, and no obvious way to get back to the light gray you'd grown comfortable with. You've already found the Office theme settings, which is more than most users ever do, but the options there don't include the exact shade you're looking for. That's not a failure on your part. That's the product making a change without asking permission, and it's reasonable to want control over something as basic as the color of your workspace.
Here's what's actually happening: the gray you remember wasn't just a theme color, it was the default appearance of the ribbon and the area around it in older versions. When Microsoft introduced the "Colorful" theme as the default in newer builds, it brought that green tint to the top and bottom of the window. Changing the Office theme to "White" should have handled most of it, but you're right that the bottom bar, the status bar, the sheet tabs, the scrollbar area, stays green. That's because those elements are tied to the accent color, not the theme. The accent color is set separately, and it's not exposed in the General options menu. You're not missing a checkbox. The setting is simply buried deeper, and for many users, it might as well not exist.
The practical path forward is to look for the "Create a new theme" or "Customize colors" option, which is usually found under the Page Layout tab in the Themes dropdown. From there, you can set the accent color to a neutral gray or even white, which will override the green in the bottom bar. It's not a one-click fix, and it's not obvious, but it works. If that feels like too much effort for something that should be simple, that's a fair reaction. The fact that a basic visual preference requires a workaround is a design miss, and we're not going to pretend otherwise.
But here's the thing we want you to take away: you're already asking the right questions. You're not settling for an interface that feels wrong. You're not accepting the default because someone at Microsoft decided green was the new gray. That curiosity, that willingness to push past the first layer of settings, is exactly the mindset that makes people effective with tools like Excel. The ribbon color is just the surface. The same persistence you're applying here will serve you when you're digging into formulas, conditional formatting, or data validation. So yes, the fix takes a few extra clicks. But you've already done the hard part: you've decided the tool should work for you, not the other way around. Now go find that accent color setting, set it to something neutral, and take back your workspace.