You can utilize built-in Bootstrap themes in there via Canvas’ admin page:
Or on what you were updating in the above, it should be in the css\style.css file and not on the app.css as this will be overwritten during upgrade.
You can also append your own css file if you want to.
Then regarding the CSS, there might be other styles in there for that ID, so try to put !important on the target style.
So the above would look like this:
width: 100% !important;
background-color: #262931 !important;
Checkout CSS topics too regarding Specificity or CSS rule style priorities for more information on these.