解决：一不小心把app/src/main/res/values/styles.xml下的内容删了，加一个即可：
<!-- Base application theme. -->
<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
<!-- Customize your theme here. -->
<item name="colorPrimary">#A000</item> <!-- 设置顶部的标题的颜色 -->
<item name="colorPrimaryDark">#8000</item> <!-- 设置标题上面的背景颜色 -->
<item name="colorAccent">#00f</item> <!-- 输入框和按钮单选框的响应颜色 -->
</style>
顺便补上colors.xml和strings.xml的内容。
string.xml：
<resources>
<string name="app_name">My Application</string>
</resources>
colors.xml:
<?xml version="1.0" encoding="utf-8"?>
<resources>
<color name="purple_200">#FFBB86FC</color>
<color name="purple_500">#FF6200EE</color>
<color name="purple_700">#FF3700B3</color>
<color name="teal_200">#FF03DAC5</color>
<color name="teal_700">#FF018786</color>
<color name="black">#FF000000</color>
<color name="white">#FFFFFFFF</color>
</resources>