Yes, you can change the fill shade of a text box in Microsoft Word to a light gray color. Here's how to do it:

Highlight the text box where you want to change the fill shade.
Click the Design tab (may be called Formatting in some versions of Word) on the tool ribbon.
On this tab, find the Fill Color or Shape Fill section (depending on your version of Word) and click on it.
From the drop-down menu, select the light gray color. It is usually found under "Light Colors" or "Gray".
Once the color is selected, the fill shade of the text box will change to light gray.