[mod] simple theme: adjust button colors in dark version.

buttons:
* previous & next in the result page
* save, restore, & back in the preferences
* back to top

<select> input in Chrom* browsers:
* fix the white text with a white background issue

searx/static/themes/simple/src/less/definitions.less

@@ -22,6 +22,7 @@
   --color-sidebar-border: #ddd;
   --color-sidebar-font: #000;
   /// BackToTop Colors
+  --color-backtotop-font: #444;
   --color-backtotop-border: #ddd;
   --color-backtotop-background: #fff;
   --color-backtotop-shadow: #ccc;
@@ -124,12 +125,13 @@
     --color-sidebar-border: #333;
     --color-sidebar-font: #fff;
     /// BackToTop Colors
+    --color-backtotop-font: #58f;
     --color-backtotop-border: #333;
     --color-backtotop-background: #181818;
-    --color-backtotop-shadow: #444;
+    --color-backtotop-shadow: #333;
     /// Button Colors
     --color-btn-background: #58f;
-    --color-btn-font: #fff;
+    --color-btn-font: #222;
     /// Search Input Colors
     --color-search-border: #444;
     --color-search-background: #222;
@@ -155,7 +157,7 @@
     --color-answer-font: #bbb; // same as --color-base-font
     --color-answer-background: #181818; // same as --color-header-background:
     /// Results Colors
-    --color-result-shadow: #444;
+    --color-result-shadow: #333;
     --color-result-border: #333;
     --color-result-url-font: #fff;
     --color-result-vim-selected: #181818;
@@ -201,7 +203,7 @@
     --color-toolkit-checkbox-label-border: #333;
     --color-toolkit-checkbox-input-border: #58f;
     --color-toolkit-engine-tooltip-border: #333;
-    --color-toolkit-engine-tooltip-shadow: #444;
+    --color-toolkit-engine-tooltip-shadow: #333;
     --color-toolkit-engine-tooltip-background: #222;
     --color-toolkit-loader-border: rgba(255, 255, 255, 0.2);
     --color-toolkit-loader-borderleft: rgba(0, 0, 0, 0);

searx/static/themes/simple/src/less/style.less

@@ -563,6 +563,13 @@ article.result-images[data-vim-selected] {
     margin: 0;
     padding: 0.7em;
   }
+
+  a,
+  a:visited,
+  a:hover,
+  a:active {
+    color: var(--color-backtotop-font);
+  }
 }
 
 #results.scrolling #backToTop {

searx/static/themes/simple/src/less/toolkit.less

@@ -338,6 +338,10 @@ select {
     &:focus {
       border-bottom: 1px solid var(--color-search-border);
     }
+
+    option {
+      background-color: var(--color-base-background);
+    }
   }
 
   @media (prefers-color-scheme: dark) {