remove backdrop-filter only if unsupported

src/components/settings_modal/tabs/theme_tab/theme_tab.js

@@ -703,6 +703,7 @@ export default {
         '&'
       ).join('\n')
 
+      sheet.clear()
       sheet.addRule('#theme-preview {\n' + rule + '\n}')
       sheet.ready = true
       adoptStyleSheets()

src/services/style_setter/style_setter.js

@@ -19,9 +19,12 @@ export const createStyleSheet = (id) => {
       this.rules = []
     },
     addRule (rule) {
+      let newRule = rule
+      if (!CSS.supports?.('backdrop-filter', 'blur()')) {
+          newRule = newRule.replace(/backdrop-filter:[^;]+;/g, '') // Remove backdrop-filter
+      }
       this.rules.push(
-        rule
+        newRule
-          .replace(/backdrop-filter:[^;]+;/g, '') // Remove backdrop-filter
           .replace(/var\(--shadowFilter\)[^;]*;/g, '') // Remove shadowFilter references
       )
     }