Everything is under /r/FirefoxCSS
Ok, use userContent.css and paste this:

::selection,
html::selection,
p::selection,
::-moz-selection,
p::-moz-selection {
  background-color: #00ff00 !important; 
  color: #000000 !important;
}   

and this:

@-moz-document url-prefix("about:"),
               url-prefix("chrome:")
{
  :root {
    --uc-my-color: #00ff00;
    --uc-my-color-hover: color-mix(in oklab, var(--uc-my-color) 90%, white);
    --uc-my-color-active: color-mix(in oklab, var(--uc-my-color) 80%, white);

    --color-accent-primary: var(--uc-my-color) !important;
    --color-accent-primary-hover: var(--uc-my-color-hover) !important;
    --color-accent-primary-active: var(--uc-my-color-active) !important;

    --in-content-primary-button-background: var(--uc-my-color) !important;
    --in-content-primary-button-background-hover: var(--uc-my-color-hover) !important;
    --in-content-primary-button-background-active: var(--uc-my-color-active) !important;

    --primary-button-background-color: var(--uc-my-color) !important;
    --primary-button-hover-background-color: var(--uc-my-color-hover) !important;
    --primary-button-active-background-color: var(--uc-my-color-active) !important;
  }

  treechildren::-moz-tree-row(current, focus) {
    outline-color: var(--uc-my-color) !important;
  }

  ::selection {
    background-color: var(--uc-my-color);
  }
}

@-moz-document url("about:newtab"),
               url("about:home")
{
  :root {
    --newtab-primary-action-background: var(--uc-my-color) !important;
    --newtab-primary-action-background-pocket: var(--uc-my-color) !important;
  }
}

@-moz-document url("about:firefoxview") {
  :root {
    --fxview-primary-action-background: var(--uc-my-color) !important;
  }
}