An Excel theme is a collection of colors, fonts, and effects that you can apply to a workbook with a couple of clicks. Themes ensure a consistent and professional look to your reports, and they let you more easily adhere to company branding and identity guidelines.
Applying an Excel Theme
You will find the themes under the “Page Layout” tab of Excel.
You can select a theme for your workbook, which would apply the collection of colors, fonts, and shape effects.
Or, you could select just a specific color or font theme to apply.
Excel features many built-in themes (and also color and font themes) that you can apply to a workbook for visual impact, saving you time trying to choose colors and fonts by hand that would work well together.
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Source: How-To Geek