You can set it in your manifest. You can globally define a theme for the whole application but you can apply other themes for activities too.
For Application:
<application
android:name=".MyApp"
android:theme="@style/Theme.global">
For Activity:
<activity
android:name=".SomeActivity"
android:theme="@style/Theme.search">
UPDATE on comment:
Then set this in your styles.xml where you define your theme attributes like this:
<item name="android:searchViewStyle">@stye/MySearchViewStyle</item>
This is the default for base Searchview:
<style name="Base.Widget.AppCompat.SearchView" parent="android:Widget">
<item name="layout">@layout/abc_search_view</item>
<item name="queryBackground">@drawable/abc_textfield_search_material</item>
<item name="submitBackground">@drawable/abc_textfield_search_material</item>
<item name="closeIcon">@drawable/abc_ic_clear_mtrl_alpha</item>
<item name="searchIcon">@drawable/abc_ic_search_api_mtrl_alpha</item>
<item name="searchHintIcon">@drawable/abc_ic_search_api_mtrl_alpha</item>
<item name="goIcon">@drawable/abc_ic_go_search_api_mtrl_alpha</item>
<item name="voiceIcon">@drawable/abc_ic_voice_search_api_mtrl_alpha</item>
<item name="commitIcon">@drawable/abc_ic_commit_search_api_mtrl_alpha</item>
<item name="suggestionRowLayout">@layout/abc_search_dropdown_item_icons_2line</item>
</style>
You can define your style like this:
<style name="MySearchViewStyle" parent="Base.Widget.AppCompat.SearchView">
<item name="searchIcon">@drawable/my_incredable_search_icon</item>
</style>