According to your description, I noticed that you only want to apply light gray to the new email background. It's suggestted that you could try to create a custom theme and apply it to your outlook client. Please refer to the following for specific operation steps:
Create a new message and set the background (New messages > Options > Page color > choose light grey);
File > Save as;
In the Save As dialog box address bar, enter %appdata%\microsoft\stationery
, and then press Enter;
The Stationery folder opens, In the File name box, enter a name for your new stationery and In the Save as type list, click HTML;
Click Save;
The message can be discarded after these steps are completed;
File > Options > Mail > Stationery and Fonts;
On the Personal Stationery tab, click Theme;
Under Choose a theme, click the theme or stationery that you created, and then click OK;
Then the next time you create a new message, it will have a light gray background by default. Hope to help you!