In some scenarios pre code/textarea elements not same size as enclosing code-input #135

Steps to reproduce: Go to https://oliver.geer.im/teclat/, click on the moon button to switch to dark mode by changing the CSS colour (in this case right now, changing the value of a CSS variable), and see the following rather than the whole code-input element being dark as expected:

The correctly-coloured portion is the extent of both the pre code and textarea elements. The mismatch in size may be due to some unusual CSS, e.g. the value of box-sizing here. When the code-input element gets typed in, the background colour but not the extent of the pre code/textarea, gets corrected, because the background colour is changed by JavaScript to match that of the pre code on each input.

(The JavaScript mentioned: needed in an earlier version but probably unnecessary now given now the pre code element resizes to the size of the code-input, but quick to run and for backwards compatibility it should be retained; mark it as deprecated in the source after checking it's not needed?).