Hi Priya,
There are many ways to achieve this:
1. Use css on your existing theme through the CustomSCSS setting in Site Administration > Appearance > Themes > Boost
There are a number of presentations linked, particularly in the Themes forum, which can show how people have done this
2. Get one of the many great, community provided, free themes from the Moodle plugins database
https://moodle.org/plugins/browse.php?list=category&id=3
There are many ways to achieve this:
1. Use css on your existing theme through the CustomSCSS setting in Site Administration > Appearance > Themes > Boost
There are a number of presentations linked, particularly in the Themes forum, which can show how people have done this
2. Get one of the many great, community provided, free themes from the Moodle plugins database
https://moodle.org/plugins/browse.php?list=category&id=3