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Issue with Nav Bar

vHere is the read only link to my website : https://preview.webflow.com/preview/minimal-tweaks?utm_medium=preview_link&utm_source=designer&utm_content=minimal-tweaks&preview=14d8937a7fc3b84d87723c64ad8d2773&mode=preview

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.