It's different from highlighting text, which has a very limited choice of colors, and doesn't update when you switch to another theme. Select the word or paragraph that you want to apply shading to. On the Home tab, in the Paragraph group, click the arrow next to Shading. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.