Is this Highlighting or Shading?
Text highlighting can be removed if you select the whole document and click the arrow on the Text Highlight Color button
(on
the Home tab) and choose the No Color option.
Shading, on the other hand, can be a little more tricky to get rid of. You can select all the text and clear all direct font formatting with Ctrl+SpaceBar and you can clear all direct paragraph formatting with Ctrl+Q.
Or you can make use of the Borders and Shading dialog box to selectively clear either shading applied to a text range or shading applied to a paragraph. Select the text; on the Home tab, click the arrow on the border button in the Paragraph group and then
click Borders and Shading. Go to the Shading tab.
As an alternative, you can get rid of all shading by running a macro.