You can either “use a different default Theme” which already has wider scroll bars, Or “customise a Theme” to have wider scrollbars if you don’t like any of the default Themes with the built-in Themes editor, Or use one of the user created Themes with wider scrollbars, Or you can just set the option to enable wider scrollbars with any default built-in Theme in Settings as per the below example.
If you have eM Client V10, you can enable wider scrollbars by going to “Menu / Settings (Preferences) / Appearance / Lists” and click the dropdown on the right of “Scrollbar Style” and select Use wide scrollbars.
If you have eM Client V8 or V9, you can change the width by editing the theme in the Theme Editor
See my post below in the following thread where to do that in V9.