Create custom interface for telegram web app bot with javascript

I'm wondering how Can WE design an interface for telegram web app bot. As mentionned in their site , there is a possibility to do that with BotApi with JavaScript , but it's not clear It's liké there are somme thèmes available and WE Can customisé them But why JavaScript is needed ? How Can WE create ower own interface with JavaScript ? Link to the section in telegram website : https://core.telegram.org/bots/webapps#color-schemes