restore mixing to support custom styles targeting Linux

[shiftkey]

With #139 removed from the linux branch, this will no longer build and emit an error like this in the output:

[WEBPACK] Errors building crash.js
Module build failed: ModuleBuildError: Module build failed: 
@include linux-context {
        ^
      No mixin named linux-context
      in /home/shiftkey/src/desktop/app/styles/ui/_scroll.scss (line 63, column 10)
    at /home/shiftkey/src/desktop/node_modules/webpack/lib/NormalModule.js:252:20
    at /home/shiftkey/src/desktop/node_modules/loader-runner/lib/LoaderRunner.js:364:11
    at /home/shiftkey/src/desktop/node_modules/loader-runner/lib/LoaderRunner.js:230:18
    at context.callback (/home/shiftkey/src/desktop/node_modules/loader-runner/lib/LoaderRunner.js:111:13)
    at Object.callback (/home/shiftkey/src/desktop/node_modules/sass-loader/lib/loader.js:76:13)
    at Object.done [as callback] (/home/shiftkey/src/desktop/node_modules/neo-async/async.js:8067:18)
    at options.error (/home/shiftkey/src/desktop/node_modules/node-sass/lib/index.js:294:32)
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

This commit should fix the build and enable the work #240 to work again in the main branch.

• test locally and confirm scrollbar styles are applied to dark theme
• wait on upstream issues with CI and installing NodeJS