[fix] simple theme: /preferences: dark theme version for select widgets

It is not possible to use CSS variable in a SVG when this is in a background.
This commit adds two .svg files, less converts them into data URL.
The two files are indentical except the fill color.

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

@@ -232,3 +232,7 @@ html {
 @icon-font-name:          "glyphicons-halflings-regular";
 //** Element ID within SVG icon file.
 @icon-font-svg-id:        "glyphicons_halflingsregular";
+
+// decoration of the select HTML elements
+@select-light-svg-path:   "../svg/select-light.svg";
+@select-dark-svg-path:    "../svg/select-dark.svg";