Navbar design

Changes: 

• Navbar:
    • New design for the navbar (opacity 1, font-weight 400, colors : grey 51 and grey 128)
    • Navbar collapses for sm screens
    • Navbar link are activated when on the corresponding page
 
• Red bar (welcome page):
    • Flat design (no transparency, no gradient)

To do:

• Change NX Logo's color (grey 51 and grey 128 instead of white)