Hi,
You should inspect the code of this line, it might as well coming from your website's css. You can inspect the css in your console (right click and choos Inspect).
The subscription form is displayed in a table, so don't expect collumns to be underneath each other on a smaller screens. In the options of the module you can select Tableless. When you choose that the module will use divs that you can control with your own css.