When changing the formatting of a spreadsheet to make it more readable, you should use light blue text on a bright blue background.

No you should not do this, as it will make it look unclear

Explanation:

At first, you should not select the bright colors for the background. And still, however, if you have selected the bright blue color as background then you need to select some other color for text, and not the light blue color for text, as light blue color over the bright blue color is certainly going to look quite unclear, and you will not be able to read properly. Thus, it will be better if you choose a light blue color as the background and white color for the text.