[Scopes] Expand arrows are black in launchpad mode

[fvsch]

The expand arrows in Scopes should use a gray fill. They look okay in the Firefox DevTools panel, but are black in launchpad mode.

The issue seems to be that we're using a Firefox-only, privileged-content-only method to pass the fill color:

.tree-node button.arrow {
  background: url("/devtools-components/images/arrow.svg") no-repeat;
  background-size: contain;
  background-position: center center;
  width: 10px;
  height: 10px;
  ...
  -moz-context-properties: fill;
  fill: var(--theme-splitter-color, #9B9B9B);
}

Also --theme-splitter-color is wrong here (not contrasted enough). Instead, we can use the new --theme-icon-dimmed-color.

[fvsch]

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