Eventually we'll add the ability to customize the popup from within Monto, but right now the only way to do this is through CSS.
The primary popup container ID is: #currencyContainer. So if you go into your Webflow project settings > custom code > go to the footer code section and paste this:
<style>
#currencyContainer {
}
</style>
You can then add any styles you want there to affect the entire container.
A list of a few potential IDs to target:
#currencyContainer
The main container and background
#currencyContainer .title
The top title
#currencyContainer .subtitle
The text under the title
#currencyContainer .presets-holder svg
The currency icon
#currencyContainer .presets-item
The currency names