Fix src/components/referenceInput/index.css file, variables doesn't exist

[osvaldovega]

Every time the app starts, there are 2 warning messages that show up saying this.

WARNING in ./node_modules/css-loader??ref--6-3!./node_modules/postcss-loader/lib??postcss!./src/components/referenceInput/index.css
(Emitted value instead of an instance of Error) postcss-custom-properties: /Users/ovega/Desktop/lisk-hub/src/components/referenceInput/index.css:5:5: variable '--search-box-line-height' is undefined and used without a fallback
 @ ./src/components/referenceInput/index.css 4:14-149 18:2-22:4 19:20-155
 @ ./src/components/referenceInput/index.js
 @ ./src/components/request/specifyRequest.js
 @ ./src/components/send/index.js
 @ ./src/components/transactionDashboard/index.js
 @ ./src/constants/routes.js
 @ ./src/components/app/index.js
 @ ./src/main.js
 @ multi (webpack)-dev-server/client?https://0.0.0.0:8080 webpack/hot/dev-server ./src/main.js

WARNING in ./node_modules/css-loader??ref--6-3!./node_modules/postcss-loader/lib??postcss!./src/components/referenceInput/index.css
(Emitted value instead of an instance of Error) postcss-custom-properties: /Users/ovega/Desktop/lisk-hub/src/components/referenceInput/index.css:6:5: variable '--search-box-font-size' is undefined and used without a fallback
 @ ./src/components/referenceInput/index.css 4:14-149 18:2-22:4 19:20-155
 @ ./src/components/referenceInput/index.js
 @ ./src/components/request/specifyRequest.js
 @ ./src/components/send/index.js
 @ ./src/components/transactionDashboard/index.js
 @ ./src/constants/routes.js
 @ ./src/components/app/index.js
 @ ./src/main.js
 @ multi (webpack)-dev-server/client?https://0.0.0.0:8080 webpack/hot/dev-server ./src/main.js

Steps to reproduce

1. run lisk hub with the command.

 npm run dev

2. check the terminal and scroll up to find the 2 warning messages.

Actual result

Looks like nothing happens in the UI except that the values set for those variables are empty.

Expected result

The variables in the css file exists with the correct value and confirm that the component has the values.

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