Align styles in Plugins view

[vitaliy-guliy]

Describe the bug

After updates in Theia upstream, elements in Che Plugins view become unreadable or got inappropriate styles.
We need to fix styles of those elements for both dark and white themes.

Steps to reproduce

1. Build Che-Theia from sources
2. Launch and open Plugins view

Runtime

• minikube (include output of minikube version and kubectl version)

Environment

• my computer
  • Linux

[azatsarynnyy]

It's fixed by eclipse-theia/theia#6475 and eclipse-che/che-theia#656
The styles now are adaptable to any Theme