All official themes are already included inside eM Client. You can view those in Menu > Settings> Appearance > Themes.

But you can also easily create your own theme from these if you are handy with XML.

Steps to follow:

• Select the theme you want to base yours on.
• Click the Advanced button and select "Save current theme as.."
  • The theme file will be the saved as a .emtheme file
• Open the .emtheme file in any text editor
• Adjust or recreate your theme using the available tags, each item is explained in the comment inside the file
• Save your theme file
• Go back to Themes settings 
• Click the Import button and import your new theme
• Select it and Apply it

You have successfully changed your eM Client theme! 

Example:

If you just want to make a small change, for example, enable wider scrollbars, find the relevant line and change the True/False value.

<UseSystemScrollbar description="Use system scrollbars with no automatic hiding">False</UseSystemScrollbar>

 Colors are saved in HEX codes, there are many sites that provide color pickers and provide you with the code - for example Color-Hex

Note: User made themes can be found on our Forum, just search for 'Theme'