hi there, doctorneutron is pretty much there, make a clear (opaque) icon, two ways to do this, either open an icon, erase all the insides then save, or jus hit create new and then save on a clear background, just make sure you save as png so it doesnt turn it all white!!
then head over to composer (keep theme builder open), select create new, and choose the transparent background (make sure your on the right device) then on the left, you have all different tabs, choose `theme button` and place it on your page, now in the `inspector` window to the right, choose `application menu` the you`ll notice theres 2 images below, normal and focus, click on each image, and select you clear png you just made, now when thats done on both, all you`ll see on the main page is the green outline of the icon area, now grab the corners of the icon, and resize it to where ya need it, note you may want to check roughly how far up from the bottom of the screen you wish to start, this is why we`ve left the theme builder open, so you can gauge what size to resize the icon to.
now thats done, all you need to do is get it into theme builder, so firstly save all your hard work (you can then just edit the size in later themes, so much quicker) the go into file> export> svg
then hit `ok` and save the file as you would any other
welldone, youve now got your svg all ready to go, just nip back into theme builder, and where youve got the long space with `svg file` written by it, click on the little folder, select your svg file that you just saved, click ok, and your done
now just sit back, ave a drink and marvel at your work