I installed OptiFine_1.12.2_HD_U_C8.jar. It added the Optifine option to the launcher, however when running the game I can see no difference?
Video settings pane does say OPTIFINE HD C8 Ultra, however the game looks like vanilla Minecraft. Changing video settings seems to have no effect.
Is there an "on" button somewhere that I'm missing?
What are you expecting to change?