Increase font size button can concatenate `20` => `202` instead of adding

[lindapaiste]

The "Increase" and "Decrease" font size buttons can have strange behavior if they are clicked after typing in the input.

To reproduce, click on the input and type 20. Then, without hitting Enter, click the "Increase" button. You should see this:

If the user hits Enter to submit the form then the "Increase" and "Decrease" buttons will work as expected.