Exclude dark css vars when light theme is turned on (#3336)

# What this PR does
Fix styling when light theme is turned on via system preferences by
excluding dark theme css vars in this case

## Which issue(s) this PR fixes
https://github.com/grafana/oncall/issues/3188

## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

CHANGELOG.md

@@ -19,6 +19,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Fix issue where Slack user connection error message is sometimes shown despite successful connection by @joeyorlando ([#3327](https://github.com/grafana/oncall/pull/3327))
 - Forward headers for Amazon SNS when organizations are moved @mderynck ([#3326](https://github.com/grafana/oncall/pull/3326))
+- Fix styling when light theme is turned on via system preferences
+  by excluding dark theme css vars in this case ([#3336](https://github.com/grafana/oncall/pull/3336))
 
 ## v1.3.57 (2023-11-10)
 

grafana-plugin/src/assets/style/vars.css

@@ -71,7 +71,7 @@
   --working-hours-shades-color-light: rgba(17, 18, 23, 0.04);
 }
 
-.theme-dark {
+.theme-dark:not(.theme-light) {
   --cards-background: var(--gray-9);
   --highlighted-row-bg: var(--gray-9);
   --disabled-button-color: hsla(0, 0%, 100%, 0.08);