fix: point to ~kubeflow/styles/fonts.scss

[DnPlas]

It seems like the path to this file recently changed by #7062, which is now preventing the centraldashboard Docker image to build wich fails with a SassError when trying to import fonts.scss. This commit points to the right path.

Error log for reference:

 #27 90.64 ./src/styles/styles.scss - Error: Module build failed (from ./node_modules/mini-css-extract-plugin/dist/loader.js):
#27 90.64 ModuleBuildError: Module build failed (from ./node_modules/sass-loader/dist/cjs.js):
#27 90.64 SassError: Can't find stylesheet to import.
#27 90.64    ╷
#27 90.64 11 │ @import '~kubeflow/lib/fonts.scss';
#27 90.64    │         ^^^^^^^^^^^^^^^^^^^^^^^^^^
#27 90.64    ╵
#27 90.64   src/styles/styles.scss 11:9  root stylesheet

See this build for more information.

[orfeas-k]

Thank you @DnPlas for the fixes. These actually make sense after #7062.
/lgtm
/approve

[google-oss-prow]

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