fix(css): improve tokens support on global css ionic files (#30364)

Issue number: internal

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## What is the new behavior?
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- Add tokens usage on global css files for Ionic theme.
- Removed global background an text variables and fixed variables names
used on `body`. This resulted in some changes on test pages colors,
hence the snapshots differences.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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Co-authored-by: ionitron <hi@ionicframework.com>

core/scripts/testing/styles.css

@@ -56,7 +56,7 @@ html.ios.ios {
 html.ionic,
 html.ionic.ios,
 html.ionic.md {
-  --ionic-global-background-color: var(--background);
+  --ion-background-color: var(--background);
   --ion-font-family: initial;
 }