Modify All the drupal font

Did you try to modify style.css of your theme?
You can add * {font-family: your-font-name;} for example.

Edit the style.css file that accompanies your theme. For example, Garland (the default theme in D5) declares its fonts in the "Generic elements" section. Change those to whatever you want, empty your browser cache (or manually reload the style.css file in your browser) and you're good to go.

After reading your post again, I wonder if you have been using <font> tags in your post and now want to change the font to something else. For that, I suggest writing your own input filter module. Perhaps the Audit module would do this, but I didn't find the demo video very explanatory and haven't tried the module yet.