Erm… HTML, CSS, Bootstrap and AngularJS. There is plenty of documentation out there on these topics. For HABPanel itselft this is a good place to start: HABPanel Development & Advanced Features: Start Here!
I would try something like this:
<h3 ng-style="{ color: parseFloat(itemState('EnBw_Aktuell')) < 0 ? 'red' : 'green' }">...</h3>
I guess “EnBw_Aktuell” is the name of the item you want to check, so you have to pass it to “itemState” (“itemValue” might also work but it’s deprecated) as a string. Also, “itemState” gives you a string, so you cant use “<” to compare it.