Revise the default value of the z-index from 100 to 999, to ensure it…

… always be shown in the front layer (#2084)

Co-authored-by: tomiir <rocchitomas@gmail.com>

packages/ui/src/utils/ThemeUtil.ts

@@ -58,7 +58,7 @@ function createRootStyles(themeVariables?: ThemeVariables) {
         --w3m-border-radius-master: ${unsafeCSS(
           themeVariables?.['--w3m-border-radius-master'] || '4px'
         )};
-        --w3m-z-index: ${unsafeCSS(themeVariables?.['--w3m-z-index'] || 100)};
+        --w3m-z-index: ${unsafeCSS(themeVariables?.['--w3m-z-index'] || 999)};
 
         --wui-font-family: var(--w3m-font-family);