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Issue with Nav Bar

vHere is the read only link to my website :

Please help me with the Navigation Bar. I want it to be fixed at top. And when I scroll down it automatically hides and when upward scrolling is done it appears back. Along with all this I want a background when navigation reaches section 2.

Please preview the speciality Section, I somehow managed to get the result but can we optimisze it? Or is there any other way to do it as it took me a lot of time.