Please refer to this section of the developer's guide:

https://sogo.nu/files/docs/SOGoDevelopersGuide.html#_defining_an_alternate_color_theme

I have copied this exact line

SOGoUIAdditionalJSFiles = (js/theme.js);

into /etc/sogo/sogo.conf. Then restarted sogo.

I was expecting the 'theme.js' file to automatically appear in this location
: /usr/lib/GNUstep/SOGo/WebServerResources/js

but it did not. So i guess, i have to manually create the 'theme.js' file.

The next problem is how to go about editing the 'theme.js' file.

Is there any instruction on how to do it ?

Have you installed packages? On which distro are you?

The file should be bundled in all packages.

Ref: https://raw.githubusercontent.com/inverse-inc/sogo/master/UI/WebServerResources/js/theme.js

But is there a particular line where i am supposed to place this code
" SOGoUIAdditionalJSFiles = (js/theme.js); "

in "/etc/sogo/sogo.conf " ?

I placed it right under the web interface section. Just exactly like this;

/ Web Interface /
SOGoUIAdditionalJSFiles = (js/theme.js);