It changes with the user theme, but you can also change the color manually or even set an image as the tab background!!

CTL+F ' <div class="card card-outline-danger" style="background-color:white;">
<div class="card-inverse card-danger card-header"> '

replace with

<div class="card card-outline" style="background-color: #HERE">
<div class="card-inverse card-header" style="background-color: #HERE;">

or if you want a background, just use this <div class="card-inverse card-header" style="background-image: url('IMAGEHERE');"> 

Edit: you can also replace it with bootstrap colors! "card card-outline-danger" I use https://toyhou.se/1099931.-coding-snippet-goldmine#4059265 this for reference, and just replace it with whichever color you'd like from here 
"card card-outline-default" for example!