The problem here is that you edit the wrong file. That T3-assets helps to keep the temporary compressed LESS/CSS files only, you should not edit any files there.
If you want to play with LESS files, you should edit files in: ROOT//templates/ja_morgan/less/ folder
Let take the topbar as sample, the style of this section declared in this file:
ROOT/templates/ja_morgan/less/style.less
At approx line 153, you will find this code:
// Topbar
.t3-topbar {
background: @topbar-bg;
color: @topbar-color;
font-size: @font-size-h6;
}
The variable like @topbar-bg is defined in ROOT/templates/ja_morgan/less/variables.less line 75:
@topbar-bg: #1b3060;
@topbar-color: rgba(255,255,255,0.5);
@topbar-height: 40px;
you can change color for variables here.
Then compile less to css: https://www.t3-framework.org/documentation/bs3-configuration#compile-less
I hope this helps.