[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/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 {