fix: Dark Mode style regressions

[bnussman-akamai]

Description 📝

• I caused some minor dark mode regressions in https://github.com/linode/manager/pull/11081
• This fixes the regressions by merging the theme options before calling createTheme

Related Docs 📖

See https://v5.mui.com/material-ui/customization/theming/#createtheme-options-args-theme

I think the key point is

Only the first argument (options) is processed by the createTheme function.

Calling createTheme before merging the raw options has implications that I didn't foresee. Calling createTheme with the theme options merged first fixes the issues we are seeing

Screenshots of the Issues 📷

How to test 🧪

• Checkout this PR (or just use the preview link)
• Verify the regressions in the "Screenshots" section are all fixed 🔧

As an Author I have considered 🤔

• [ ] 👀 Doing a self review
• [ ] ❔ Our contribution guidelines
• [ ] 🤏 Splitting feature into small PRs
• [ ] ➕ Adding a changeset
• [ ] 🧪 Providing/Improving test coverage
• [ ] 🔐 Removing all sensitive information from the code and PR description
• [ ] 🚩 Using a feature flag to protect the release
• [ ] 👣 Providing comprehensive reproduction steps
• [ ] 📑 Providing or updating our documentation
• [ ] 🕛 Scheduling a pair reviewing session
• [ ] 📱 Providing mobile support
• [ ] ♿ Providing accessibility support