Thank you Zet Man, I will leave it for a while as I have no wish to run into more problems this week. I find it stressful when I edit, upload and get a white screen. I appreciate your help and your team.

The procedure has been discussed already many times.

• From within your browser move the cursor to the element of interest
• Then right-click Inspect (Element)
  With Firefox you'll get a screen like that
  
• Next, search btn btn-warning in the source (see https://www.opensource-socialnetwork.org/wiki/view/1137/how-to-find-a-string-a-pattern-of-words-in-the-source-code)
• And you'll find the file system\plugins\default\pages\contents\index.php

Since it's no good idea to change any CORE file and you have made a lot of other changes already anyway, I'd recommend to clone your current theme (see https://www.opensource-socialnetwork.org/wiki/view/1063/how-to-develop-a-new-theme) and copy the index.php to the corresponding location (plugins\default\pages\contents\) in your own theme and apply the different color there. Read the bootstrap docs for available other colors - if you don't like them, create your own class and add it to the css of your theme.