feat(dashboard): change chart background option from "White" to "Solid" (#34655)

Co-authored-by: Claude <noreply@anthropic.com>

superset-frontend/src/dashboard/components/menu/BackgroundStyleDropdown.tsx

@@ -43,11 +43,11 @@ const BackgroundStyleOption = styled.div`
       display: inline-block;
       vertical-align: middle;
     }
-    &.background--white {
+    &.background-style-option.background--white {
       padding-left: 0;
       background: transparent;
       &:before {
-        background: ${theme.colors.grayscale.light5};
+        background: ${theme.colorBgContainer};
         border: 1px solid ${theme.colorBorder};
       }
     }

superset-frontend/src/dashboard/util/backgroundStyleOptions.ts

@@ -27,7 +27,7 @@ export default [
   },
   {
     value: BACKGROUND_WHITE,
-    label: t('White'),
+    label: t('Solid'),
     className: 'background--white',
   },
 ];