When sitting at a light at idle, the headlight goes dim. Then when I rev up and drive normally, the light go up to normal.

The light gets brighter because the higher crank speed is generating more electricity. Your battery is getting weak. Check to see that water covers the top of the plates in each battery cell. A strong battery will make up the difference at idle speeds. Most bike batteries will only last two years or so. Please rate my answer. Thanks.