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

In eM Client 8 we introduced a new Theme editor, where you can use a direct WYSIWYG editor.

You can find all steps and tips in this article - Design your own eM Client

Don't have the time to design your own theme right now but the official themes don't fit your style?

You can also choose from our user-created Theme gallery.

After creating or downloading your new theme, you still need to Import it in Menu > Settings> Appearance > Themes and then select it from the Theme list to apply it.

LEGACY OPTIONS

Steps to follow for eM Client 7 and older:

• 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 for direct XML edit:

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'