[🚀 site]: add site theme color

[dshovchko]

As an ESL core team member I want the site to use the theme color. Mobile user agents use this color to customize the display of the page and the surrounding user interface.

Right now, the page on mobile devices looks something like this (the color at the top depends on the user's theme)

After adding a theme, it will look like this

MDN: manifest theme_color