You can use CSS styles to control the appearance of elements in a theme.

Your version of the UI determines which CSS styles you can use.

Default CSS styles for Core UI (UI16)

$search-text-color: #ffffff                         
$navpage-header-bg: #293e40                         
$navpage-header-color: #ffffff
$navpage-header-divider-color: #5A7F71
$navpage-header-button-color: #ffffff          
$nav-highlight-main: #576667                        
$subnav-background-color: #213234                   
$navpage-nav-bg: #293e40                              
$navpage-nav-bg-sub: #293e40                           
$navpage-nav-color-sub: #d1d6d8                                           
$nav-hr-color: #192628                              
          
$nav-highlight-bar-active: #82c9b8                  
$nav-highlight-bar-inactive: #213234                
$navpage-nav-selected-bg: #2f4f4e                   
$navpage-nav-selected-color: #82c9b8                  
$navpage-nav-unselected-color: #d1d6d8              
          
$connect-latest-message: #B8E0D7                                     
$navpage-nav-app-text: #ffffff                                        
$navpage-nav-border: #7a828a                        
$navpage-button-color: #ffffff  
$navpage-button-color-hover: #82C9B8