Looks very cool. It is a little hard to find the right currency, so I think it might be a good idea to sort the list of currencies, probably alphabetically, but maybe want the common ones (USD, EUR, GBP, etc.) at the top, above an
<hr /> tag.
Regarding the styling, I would probably add a bit of
padding-top as it is quite close to the top-left edge of the screen on Desktop. On Mobile, it looks good, but I think I would look better if it was centred.
If you wanted to go further you could add a slide rule, and keyboard up and down buttons to choose the value and also add a button to switch the values (i.e. if you are converting USD to AUD, with one click you can change to convert AUD to USD).