Basically you can set border-style of .rtTop/.rtMid/.rtBot to dashed to get dashed borders:
div.RadTreeView .rtLines .rtTop,
div.RadTreeView .rtLines .rtMid,
div.RadTreeView .rtLines .rtBot
You will also need to remove the padding of .rtUL and move it to .rtLI:
div.RadTreeView .rtUL .rtLI .rtUL
div.RadTreeView .rtLI .rtLI
Let me know if this helps.
If you can't do it yourself, you can also send me your custom skin in a support ticket and I will try to help you fix it.
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