Update slider handle color based on value.

Responsive Slider Handle Color.ipynb

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   "source": [
    "from ipywidgets import IntSlider, interact\n",
    "# Compare https://github.com/jupyter-widgets/ipywidgets/blob/master/docs/source/examples/Widget%20Styling.ipynb "
   ]
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   "source": [
    "s1 = IntSlider(description='Drag Me')\n",
    "s1.style.handle_color = 'silver'"
   ]
  },
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   "cell_type": "code",
   "execution_count": 3,
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     "data": {
      "application/vnd.jupyter.widget-view+json": {
       "model_id": "3484398951cf40cebeab620e52b8b492",
       "version_major": 2,
       "version_minor": 0
      },
      "text/plain": [
       "interactive(children=(IntSlider(value=0, description='Drag Me', style=SliderStyle(handle_color='silver')), Out…"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "@interact(slider=s1)\n",
    "def react(slider):\n",
    "    s1.style.handle_color = 'lightgreen' if slider < 50 else 'tomato'\n",
    "    print(slider)"
   ]
  },
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   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": []
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Interactive Demo:

Compare Widget Styling Notebook: https://github.com/jupyter-widgets/ipywidgets/blob/master/docs/source/examples/Widget%20Styling.ipynb