I am using elementary OS which is based on Ubuntu Linux. I would like to change my fonts everywhere text is displayed on OS. Is there a way to do it, or am I too hopeful?
You can install the application dconf-editor with your terminal and
there you can change these setting. Type in your terminal...
sudo apt install dconf-editor
Then, after installation finished open dconf-editor and go to
org > gnome > desktop > interface
Now on the right side there is a point called 'font-name' and after that its defined with 'Open Sans 9' by default. This is your default system font.
Change the number to 10 for example and you see the affect immediately.
The font for terminal or scratch-text-editor are separately.
For terminal go to...
org > pantheon > terminal > settings
For scratch-text-editor got to...
org > pantheon > scratch > settings