Officelite coming soon site using React .js

Hi, this was my first project, with REACT-ROOTER-DOM and I will appreciate if you would like to give me some advice on its use.

1. On my second page I created two sections and used a BACKGROUND-IMG for my second section to put an img. I used display flex and directional row and column to switch between desktop screen and mobile screen, but my problem is that I can't use height: 100% or width 100%, I have had to use the px or vh. This is not good when page scroll, how can I improve it?
2. how to add a css style on the <option> balise in the <select> balise or should you use css frameworks?