@mauryeel: Replace the previous css with the following. Make sure, you copy all 4 lines of CSS:
.site-branding { width: 100%; text-align: center; }
.site-branding .header-site-details { float: none; padding: 0; }
#site-navigation { clear: both; float: none; margin: 0; text-align: center; }
#site-navigation ul li { float: none; display: inline-block; }
Then for the title, remove the all caps, you can add the following css:
.site-branding .site-title { text-transform: none; }
Finally for the font size changes, I am not going to change that for mobile. So, you can change the font size in the following css as per your need and add it in “Appearance => Customize => Additional CSS”
@media screen and (min-width: 990px) {
.site-branding .site-title { font-size: 28px; }
.site-branding .site-description { font-size: 14px; }
}