Microsoft annoyingly uses the 'Outlook' label for different things. So it's not clear from your post as to whether you're using the dedicated Outlook app or viewing the Outlook.com website via your web browser.
Either way, start by going to Settings and then to Apps, followed by App Settings. (If you're using a Samsung device, that'll probably be labelled as 'Samsung app settings. There might be other small differences across devices too). It Outlook is listed there, tap on it and look for a way to disable 'Dark theme'. If it's not, tap on the name of your browser (which might be Chrome or, if you've got a Samsung device, Samsung Internet) and similarly look for a way to disable 'Dark theme'.
(Android is a fantastic operating system. Indeed, in many ways it's far better than Windows. However the fact that each manufacturer can tweak it to their own way of doing things makes providing instructions for Android devices a bit of a minefield. What works on one manufacture's device might well not do so one a device from a different manufacturer. So I'm fumbling in the dark here!).