Remove illegal usages of CSS variables in SCSS functions

This flattens out the buttons on the settings page which makes the styles simpler (and more modern-looking IMO) and removes the box-shadow on the button, which was completely invisible to me.

assets/stylesheets/components/_content.scss

@@ -416,14 +416,12 @@
   line-height: normal;
   white-space: nowrap;
   padding: .375rem .675rem;
-  background-image: linear-gradient(lighten(var(--boxBackground), 4%), darken(var(--boxBackground), 2%));
+  background-color: var(--boxBackground);
   border: 1px solid var(--boxBorder);
   border-radius: 3px;
-  box-shadow: 0 1px rgba(var(--boxBorder), .08);
   cursor: pointer;
 
   &:active {
-    background-color: var(--boxBackground);
     box-shadow: inset 0 1px 1px rgba(black, .05), inset 0 1px 4px var(--boxBorder);
   }
 }

assets/stylesheets/components/_header.scss

@@ -164,7 +164,7 @@
   font-size: .875rem;
   background: var(--contentBackground);
   border: 1px solid;
-  border-color: darken(var(--headerBorder), 2%);
+  border-color: var(--searchBorder);
   border-radius: 3px;
 
   &:focus {

assets/stylesheets/global/_variables-dark.scss

@@ -28,6 +28,7 @@ html.dark {
   --headerBackground: #1c1c1c;
   --headerBorder: #000;
   --searchTagBackground: darken(#1c1c1c, 5%);
+  --searchBorder: darken(#000, 2%)
 
   --sidebarBackground: #24282a;
   --transparentSidebarBackground: rgba(#24282a, 0);

assets/stylesheets/global/_variables-light.scss

@@ -28,6 +28,7 @@ html:not(.dark) {
   --headerBackground: #eee;
   --headerBorder: #d7d7d7;
   --searchTagBackground: darken(#eee, 5%);
+  --searchBorder: darken(#d7d7d7, 2%)
 
   --sidebarBackground: #f9f9f9;
   --transparentSidebarBackground: rgba(#f9f9f9, 0);