Making TabLayout text bold

1. One option is to add In styles.xml

     <item name="android:textStyle">bold</item> 
   

   inside "TextAppearance.Design.Tab" with same name as parent

   <style name="TextAppearance.Design.Tab" parent="TextAppearance.Design.Tab">
     <item name="android:textSize">15sp</item>
     <item name="android:textStyle">bold</item>
     <item name="android:textColor">?android:textColorSecondary</item>
     <item name="textAllCaps">true</item>
     <item name="android:singleLine">true</item>
   </style>
   

2. Other option : inside your layout direct to your style - lets say you call it myTabLayoutStyle

    style="@style/myTabLayoutStyle"
   

and inside that style redirect again to other style just for text appearance :

      <item name="tabTextAppearance">@style/myTabTextStyle</item>

like that:

  <android.support.design.widget.TabLayout
    android:id="@+id/tab_layout"
    style="@style/myTabLayoutStyle"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_below="@+id/toolbar"
    android:background="?attr/colorPrimary"
    android:elevation="600dp"
    android:minHeight="?attr/actionBarSize"
    app:tabGravity="fill"
    android:singleLine="true"
    android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"/>

inside styles.xml:

  <style name="myTabLayoutStyle" parent="Widget.Design.TabLayout">
    <item name="tabMaxWidth">@dimen/tab_max_width</item>
    <item name="tabIndicatorColor">?attr/colorAccent</item>
    <item name="tabIndicatorHeight">4dp</item>
    <item name="tabPaddingStart">3dp</item>
    <item name="tabPaddingEnd">3dp</item>
    <item name="android:singleLine">true</item>
    <item name="tabBackground">?attr/selectableItemBackground</item>
    <item name="tabSelectedTextColor">?android:textColorPrimary</item>
    <item name="tabTextAppearance">@style/myTabTextStyle</item>
  </style>

   <style name="myTabTextStyle">
       <item name="android:textSize">15sp</item>
       <item name="android:textStyle">bold</item>
       <item name="android:textColor">?android:textColorSecondary</item>
       <item name="textAllCaps">true</item>
       <item name="android:singleLine">true</item>
  </style>

You need to declare the following styles

<style name="TabLayoutTextStyle">
    <item name="android:textSize">16sp</item>
    <item name="android:textStyle">bold</item>
    <item name="android:textColor">@color/black</item>
</style>

Now you can just use it like:

Then the style must be applied to the TabLayout using app:tabTextAppearance not the style attribute!

<android.support.design.widget.TabLayout
     android:layout_width="match_parent"
     app:tabTextColor="@color/white"
     app:tabSelectedTextColor="@color/white"
     app:tabIndicatorColor="@color/accent"
     android:layout_height="wrap_content"
     app:tabTextAppearance="@style/TabLayoutTextStyle" />

First this must be added to the styles.xml:

<style name="TabLayoutTextStyle">
    <item name="android:textSize">16sp</item>
    <item name="android:textStyle">bold</item>
</style>

Even if you don't want to alter the text size, you must include it in the styles otherwise nothing will be shown.

Then the style must be applied to the TabLayout using app:tabTextAppearance not the style attribute!

<android.support.design.widget.TabLayout
android:id="@+id/sliding_tabs"
android:layout_width="match_parent"
app:tabTextColor="@color/white"
app:tabSelectedTextColor="@color/white"
app:tabIndicatorColor="@color/accent"
android:layout_height="wrap_content"
app:tabIndicatorHeight="3dp" 
app:tabTextAppearance="@style/TabLayoutTextStyle" />

To enable the allcaps you may add the following to the TabLayoutTextStyle:

<item name="android:textAllCaps">true</item>