Administration: Improve the usage of the button CSS classes.

Introduces some consistency in the usage of the button CSS classes, fixes the
focus style for accessibility and responsiveness of the buttons.

- Adds the `button` class to all primary buttons make them responsive
- Removes all `secondary-button` classes and replaces it with button when needed. `button-secondary` shouldn't be used and exists just for backward compatibility reasons
- Replaces classes inside `submit_button()` with a shorthand for some buttons, and use an empty string for the default `button` class. Passing `button` is unnecessary
- Adjusts `get_submit_button()` to remove empty items

Props iseulde, dimchik, chris_d2d, mhowell, afercia.
Fixes #27314, #37138, #37448.


git-svn-id: https://develop.svn.wordpress.org/trunk@38672 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/theme-install.php

@@ -85,7 +85,7 @@ function install_theme_search_form( $type_selector = true ) {
 	<label class="screen-reader-text" for="s"><?php _e('Search by keyword'); ?></label>
 	<?php endif; ?>
 	<input type="search" name="s" id="s" size="30" value="<?php echo esc_attr($term) ?>" autofocus="autofocus" />
-	<?php submit_button( __( 'Search' ), 'button', 'search', false ); ?>
+	<?php submit_button( __( 'Search' ), '', 'search', false ); ?>
 </form>
 <?php
 }
@@ -130,7 +130,7 @@ function install_themes_dashboard() {
 
 </div>
 <br class="clear" />
-<?php submit_button( __( 'Find Themes' ), 'button', 'search' ); ?>
+<?php submit_button( __( 'Find Themes' ), '', 'search' ); ?>
 </form>
 <?php
 }
@@ -145,7 +145,7 @@ function install_themes_upload() {
 	<?php wp_nonce_field( 'theme-upload' ); ?>
 	<label class="screen-reader-text" for="themezip"><?php _e( 'Theme zip file' ); ?></label>
 	<input type="file" id="themezip" name="themezip" />
-	<?php submit_button( __( 'Install Now' ), 'button', 'install-theme-submit', false ); ?>
+	<?php submit_button( __( 'Install Now' ), '', 'install-theme-submit', false ); ?>
 </form>
 	<?php
 }