[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/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) {