Yes, you can create your own .css file and attach it using the Resources section of the in5 dialog. Here are 2 different CSS code snippets that will shrink the arrows to half size for example...
Note: In newer versions of in5 (v3.1+), the navigation arrows are standard to all page formats and accessed with the following CSS:
nav#page-nav > button { }
nav#page-nav > button:hover { }
nav#page-nav > #backBtn { }
nav#page-nav > #nextBtn { }
A) This ways is a bit easier but it relies on the browser supporting CSS transforms.
.anythingSlider-in5 .arrow a {
-ms-transform: scale(0.5, 0.5); /* IE 9 */
-webkit-transform: scale(0.5, 0.5); /* Chrome, Safari, Opera */
transform: scale(0.5, 0.5);
}
OR
B) This way involves a bit more tweaking, and is also not supported in IE8 due to the
background-size property.
.anythingSlider-in5 .arrow a {
background-size: 200% auto;
height: 34px;
margin: -34px 10px 0;
width: 34px;
}
It may be easier to make tweaks to the CSS while you're testing using the
browser inspector.