Removed hard-coding of light blue in plotter.jsx

Test Plan:

- Check out the plotter [here](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20plotter%201%5D%5D%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22plotter%201%22%3A%7B%22type%22%3A%22plotter%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22correct%22%3A%5B1%5D%2C%22starting%22%3A%5B1%5D%2C%22type%22%3A%22bar%22%2C%22labels%22%3A%5B%22%22%2C%22%22%5D%2C%22categories%22%3A%5B%22%22%5D%2C%22scaleY%22%3A1%2C%22maxY%22%3A10%2C%22snapsPerLine%22%3A2%2C%22labelInterval%22%3A1%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22hints%22%3A%5B%5D%7D)
- Notice that the colors remain unchanged