Add this CSS

/* -- START styling Queen Of Hearts BOB Opt-in fields -- */
.hws-qoh-bob {
	background-color: #FFFFFF !important;
	border-color: #ffd102 !important;
	width: 100% !important;
	border-radius: 3px !important;
	font-size: 15px;
	font-family: inherit;
	color: #6b4128 !important;
	padding: 6px !important;
	-moz-box-sizing: border-box;
	-webkit-box-sizing: border-box;
	box-sizing: border-box;
	}
/* -- END styling Queen Of Hearts BOB Opt-in fields -- */Code language: CSS (css)

Then add the class to each input field:

<input class="hws-qoh-bob" type="text" name="Voornaam" id="Voornaam" />Code language: JavaScript (javascript)

And for the button

<p>
<div><input class="et_pb_button et_pb_custom_button_icon" type="submit" name="sign_up" value="Aanmelden" /></div></form>
</p>Code language: HTML, XML (xml)