To change the appearance like the text fonts, colours, or general look of objects in all worksheets of your workbook quickly, try switching to another theme or customizing a theme to meet your needs. If you like a specific theme, you can make it the default for all new workbooks.
Before the start, do have a look on “What’s new in Excel 2019 for Windows“
To switch to another theme, click Page Layout > Themes, and pick the one you want.
To customize that theme, you can change its colors, fonts, and effects as needed, save them with the current theme, and make it the default theme for all new workbooks if you want.
Change the appearance by theme colours
Picking a different theme colour palette or changing its colours will affect the available colours in the colour picker and the colours you’ve used in your workbook.
1. Click Page Layout > Colors, and pick the set of colours you want.
The first set of colours is used in the current theme.
2. To create your own set of colours, click Customize Colors.
3. For each theme colour, you want to change, click the button next to that colour, and pick a colour under Theme Colors.
To add your own colour, click More Colors, and then pick a colour on the Standard tab or enter numbers on the Custom tab.
Tip: In the Sample box, you get a preview of the changes you made.
4. In the Name box, type a name for the new colour set, and click Save.
Tip: You can click Reset before you click Save if you want to return to the original colors.
5. To save these new theme colours with the current theme, click Page Layout > Themes > Save Current Theme.
Change the appearance by using theme fonts
Picking a different theme font lets you change your text at once. For this to work, make sure Body and Heading fonts are used to format your text.
1. Click Page Layout > Fonts, and pick the set of fonts you want.
The first set of fonts is used in the current theme.
2. To create your own set of fonts, click Customize Fonts.
3. In the Create New Theme Fonts box, in the Heading font and Body font boxes, pick the fonts you want.
4. In the Name box, type a name for the new font set, and click Save.
5. To save these new theme fonts with the current theme, click Page Layout > Themes > Save Current Theme.
Change the appearance by using theme effects
Picking a different set of effects changes the look of the objects you used in your worksheet by applying different types of borders and visual effects like shading and shadows.
1. Click Page Layout > Effects, and pick the set of effects you want.
The first set of effects is used in the current theme.
Note: You can’t customize a set of effects.
2. To save the effects you selected with the current theme, click Page Layout > Themes > Save Current Theme.
Save a custom theme for reuse
After making changes to your theme, you can save it to use it again.
1. Click Page Layout > Themes > Save Current Theme.
2. In the File name box, type a name for the theme, and click Save.
Note: The theme is saved as a theme file (.thmx) in the Document Themes folder on your local drive and is automatically added to the list of custom themes that appear when you click Themes.
Use a custom theme as the default for new workbooks
To use your custom theme for all new workbooks, apply it to a blank workbook and then save it as a template named Book.xltx in the XLStart folder (typically C:\Users\user name\AppData\Local\Microsoft\Excel\XLStart).
To set up Excel so it automatically opens a new workbook that uses Book.xltx:
1. Click File>Options.
2. On the General tab, under Startup options, uncheck the Show the Start screen when this application starts box.
The next time you start Excel, it opens a workbook that uses Book.xltx.
Tip: Pressing Ctrl+N will also create a new workbook that uses Book.xltx.
If you have any doubt, feel free to contact.