In order to change the theme or highlighted color of EditText do the following steps:-
1) Create colors.xml file inside the values folder (if you don't have) and then declare colors

<color name="ColorPrimary">#4484F6</color>
<color name="colorControlNormal">#c5c5c5</color>

2) Go to styles.xml and do modification in your application theme

<!-- Base application theme. -->
    <style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
        <!-- Customize your theme here. -->
        <item name="colorControlNormal">@color/colorControlNormal</item>
        <item name="colorControlActivated">@color/ColorPrimary</item>
        <item name="colorControlHighlight">@color/ColorPrimary</item>
    </style>

Now run your application, you can see that your EditText theme is changed according to your requirement.