Thank you for your help guys.
Ok in the past days I worked in a code and make a lot of changes and finally found the 2 bugs that are causing the problems.
The first bug that I fixed is with the texture creation and with the white background. ( As you said. )
Yeah, using the System.Drawing library I lose the cross-platform option so I started to make a new way to create a texture from a pointer. This is a cross-platform code and working with no bug. 
Setting The Sampler to PointClamp and the White background to black is resolve the font textures problems.
