Hi @ankush981
Thank you for your question!
At this stage, it isn't possible to change the web UI of Mattermost using settings from within the software. You are able to change the font, theme and profile pic.
If you fork the branch to your own repo then, of course, you will then be able to make changes... If you're looking for a more technical answer, let me know and I'll ask one of our devs to assist!
Thanks!