Ensure common styles for app theme don't leak to consumers

[AlanGreene]

Changes

Scope the light theme to ensure it only affects the Dashboard app, rather than applying to all consumers who haven't set the tkn--theme-dark class on some wrapper. Consumers may choose to use a different theme, e.g. white as their light theme so we need to make sure this is respected.

/kind misc

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• [ ] Docs included if any changes are user facing
• [ ] Tests included if any functionality added or changed
• [ ] Follows the commit message standard
• [ ] Meets the Tekton contributor standards (including functionality, content, code)
• [ ] Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
• [ ] Release notes block below has been updated with any user facing changes (new features, significant UI changes, API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• [ ] Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: briangleeson

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/tektoncd/dashboard/blob/main/OWNERS)~~ [briangleeson] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment