Those issues would have been noticed if they were bugs, and there's a distinct lack of posts reporting the issue. So it's more likely the theme you're using is buggy or corrupt. Either that or your video card or RAM is dying a slow death.
Mint Testing Team & Mint Donor #3606
KDE 4.12.0, custom preemptive kernel 3.12.5,
Intel i7 4770K @ 4.7GHz, 16GB 2666MHz XMP,
4 Samsung 840 PRO 512GB SSDs in RAID0,
6TB HW RAID10, dual 24" Acer X243H,
Gigabyte nVidia GTX 680 Super Overclock