You can use any theme and customise it on your own website without running into any problems. There are many free themes available and also many paid options. You will only have problems if you try to obtain and use a paid theme without actually paying for it. You can not modify a theme and then re-sell it as your own property either unless the theme you are developing from allows this in its terms and conditions. Most theme developers exclude this as a possibility, but some do allow others to build off their work.