An Excel theme is a collection of colors, fonts, and effects that you can apply to a workbook in a few clicks. Designs ensure a consistent and professional look and feel of your reports, and make it easier to comply with corporate brand and identity policies.
Applying an Excel Theme
You can find the themes on the Page Layout tab of Excel.
tab You can select a topic for your workbook that will apply the collection of colors, fonts, and shape effects.
Or You can also select only a specific color or font theme for the application.
Excel includes many built-in themes (as well as color and font themes) that you can apply to a workbook to enhance and save the visual impact. You must try to manually select colors and fonts that fit together well.
The following image shows the blue-green color scheme applied to a workbook. Both diagrams adopt the color theme and maintain a consistent appearance.
How to Create a Custom Theme
That's great! However, the ability to create a custom theme is exactly what you need.
Creating a Custom Color Theme
Click the Colors list in the Themes group and click Customize Colors at the bottom of the list. 
This will open the Create New Theme Colors window.
Enter a name for your new color theme. In this example, I'll create a color scheme for the marketing team.
Next, select the colors you want for this topic from the lists and click Save when you're done.
This color theme is then available in the list of options that can be applied to the workbook.
The following picture shows the applied topic and its effects on the diagrams and also on the cells behind the list.
The theme applied also affects the options you receive when applying colors to other sections of Excel. For example, you can see the color picker that is now available when applying a fill color.
The options are variations of the colors that I selected in the marketing topic.  Creating a Custom Font Design
Click the Font button, and then click Customize Font.
The Edit Font Design window opens.  Enter a name for your font design and select from the lists the fonts you want to use for the "headings" and "text font". Click on "Save".
This font theme is then available in the Fonts list and is applied to the workbook below.
The Effect Design
The Effect Design is a selection of shadows, reflections, lines, and other effects that can be applied to shapes.
You can not customize these themes, but you can choose one from the list available through the Effects button.
Saving Your Custom Theme
You can use the custom color and font themes that were created and applied to save them to a topic.
Click the Themes button and then click Save Current Theme.
The Save Current Topic window opens. A theme is a THMX file and can be saved like an Excel workbook.
Enter a file name for the theme.
The default location is in the template folder. Excel automatically searches for the topics here. It is therefore recommended to save them here. You can, however, save a topic anywhere. Click on "Save".
You can now apply the theme to any workbook using the Themes button.
If you saved the topic to a location other than the template folder, click Search for Topics to locate it.
Click a button to display all colors and fonts, and effects are applied to the workbook. This can save hours of formatting time.