Change table zebra-stripe color to Wonder Blocks offWhite (#908)

## Summary:
This ensures that color-coded math expressions have accessible contrast
with the darker table rows.

Issue: https://khanacademy.atlassian.net/browse/LC-1577

Test plan:

Run storybook: `yarn storybook`.

Visit
`http://localhost:6006/?path=/story/perseus-renderers-item-renderer--label-image-item`

You should see a table of dog breeds. The background color of the
odd-numbered rows should be `#f7f8fa`.

<img width="230" alt="Screen Shot 2024-01-03 at 3 44 35 PM" src="https://github.com/Khan/perseus/assets/693920/834c0e30-dadd-4787-9b36-35e692082c29">

Author: benchristel

Reviewers: handeyeco, benchristel

Required Reviewers:

Approved By: handeyeco

Checks: ✅ codecov/project, ✅ codecov/patch, ✅ Upload Coverage, ✅ Publish npm snapshot (ubuntu-latest, 20.x), ✅ Extract i18n strings (ubuntu-latest, 20.x), ✅ Publish Storybook to Chromatic (ubuntu-latest, 20.x), ✅ Check builds for changes in size (ubuntu-latest, 20.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 20.x), ✅ Cypress (ubuntu-latest, 20.x), ✅ gerald, ✅ Jest Coverage (ubuntu-latest, 20.x)

Pull Request URL: #908

.changeset/dry-mugs-camp.md

@@ -0,0 +1,10 @@
+---
+"@khanacademy/perseus": patch
+---
+
+Lighten the background color for odd-numbered rows of Markdown tables on
+desktop-sized screens. Previously, the color was `#ededed`; it is now
+`#f7f8fa` (Wonder Blocks offWhite).
+
+This change ensures that color-coded math expressions will have accessible
+contrast with the background.

packages/perseus/src/styles/perseus-renderer.less

@@ -314,7 +314,7 @@
         tr:nth-child(odd) {
             // Doesn't work in IE8 :(
             td {
-                background-color: #ededed;
+                background-color: @offWhite;
             }
         }
     }

packages/perseus/src/styles/variables.less

@@ -15,6 +15,7 @@
 // TODO(charlie): DRY this up. How can we keep our global palette in sync across
 // repos?
 @white: #ffffff;
+@offWhite: #f7f8fa;
 @gray98: #fafafa;
 @gray97: #f6f7f7;
 @gray95: #f0f1f2;