These are the steps:
1) Get the font file you want to use.
2) Make three copies of the font.
3) Now rename these three font files to DroidSans.ttf,
DroidSans-Bold.ttf, and DroidSansMono.ttf
4) Now connect your phone to the computer using mass storage mode.
5) Go to E:/System/Apps/Opera Mobile/fonts folder
6) Copy the three font files you have renamed to this folder. and you are done.
7) If you require to change the font of the opera mobile app system font you have to rename the required font file to DroidSansFallback.ttf and copy it to the following folder E:/System/Apps/Opera Mobile/fonts2 and you are done.