[mod] simple theme: use :root instead of html to define CSS variables

This allows to use the CSS variables in inline svg.

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

@@ -5,7 +5,7 @@
  */
 
 /// Light Theme
-html {
+:root {
   /// Base Colors
   --color-base-font: #444;
   --color-base-font-rgb: 68, 68, 68;
@@ -106,7 +106,7 @@ html {
 
 /// Dark Theme (autoswitch based on device pref)
 @media (prefers-color-scheme: dark) {
-  html {
+  :root {
     /// Base Colors
     --color-base-font: #bbb;
     --color-base-font-rgb: 187, 187, 187;