[Refactor] Reduce usage of euiThemeVars

[joshuarrrr]

There are multiple ways that OSD files can integrate with OUI themes. But as much as possible, theme-specific logic should be avoided in OSD js files. In order of preference:

1. Use OUI components (which automatically take care of theming
2. When custom styles are necessary, use OUI SASS variables in SCSS
3. As a last resort, use the themed JSON variables (accessed via euiThemeVars from @osd/ui-shared-deps/theme)

Option 3 should be reserved for cases where we're relying on 3rd-party libraries that require styling via JS. We should refactor other usage to Option 2:

• https://github.com/opensearch-project/OpenSearch-Dashboards/blob/main/src/plugins/expressions/public/react_expression_renderer.tsx - This can be re-written as classes and SCSS styles.
• https://github.com/opensearch-project/OpenSearch-Dashboards/blob/main/src/plugins/opensearch_dashboards_react/public/code_editor/editor_theme.ts - Because ui-shared-deps already handles the job of switching between light and dark mode themes, it should be possible to just define a single theme, and remove the hard-coded background color in favor of the appropriate theme variable.
• https://github.com/opensearch-project/OpenSearch-Dashboards/blob/main/src/plugins/vis_type_vislib/public/vislib/components/tooltip/tooltip.js. - The dynamic size calculations can likely be refactored to SCSS.