fix: make sidebar options visible in dark themes (#371)

- Remove styles from shell.component.scss and sidenav.component.scss which are overriding the styles provided by angular material theme scss file.

Fixes: #366

src/app/core/shell/shell.component.scss

@@ -3,7 +3,6 @@
   height: 100vh;
 
   .sidebar-panel {
-    background-color: #fff;
     min-height: 100vh;
     overflow: auto;
     box-shadow: 0 3px 1px -2px rgba(0, 0, 0, 0.2), 0 2px 2px 0 rgba(0, 0, 0, 0.14), 0 1px 5px 0 rgba(0, 0, 0, 0.12);

src/app/core/shell/sidenav/sidenav.component.scss

@@ -4,7 +4,6 @@
     width: 100%;
     height: 100%;
     padding-top: 20%;
-    background: hsla(0, 0%, 100%, 0.95);
   }
 
   img {