The font that you see on a website or an application is specific to it (unless, there's an override setting) and, usually, is not controlled by your browser or system font settings.
Please check the settings menu for an option to change the font. I guess, it's under <Application_Menu> → Settings, like Steam → Settings, for example.
If the font setting option menu is not present, you have to change them under the config files from the terminal. Since you haven't mentioned your OS type, I have to assume that it's Ubuntu, as you've placed this question in askubuntu.com.
Usually, the style configuration files will be under a folder named resources or configuration that is present inside the software installation path.
For example, /opt/steam/resources/style or /usr/share/steam/resources/style or /opt/steam/config.
The sub-folder name could be style or skins - depending on the browser/application.
Now, check for a file that has the font (or, skin) configuration in it and change the font-type in it, according to your taste (you need to have font-family name ready, which you can get from your installed fonts in the system).
Be careful not to change any other setting under the config folders - unless you are sure of the impact. Also, it's always wise to make a backup of the file that you're editing before making the changes.
Note: The folder names given above are just a guideline of the pattern to look for & not the exact folders, themselves.
The font that you see on a website or an application is specific to it (unless, there's an override setting) and, usually, is not controlled by your browser or system font settings.
Please check the settings menu for an option to change the font. I guess, it's under <Application_Menu> → Settings, like Steam → Settings, for example.
If the font setting option menu is not present, you have to change them under the config files from the terminal. Since you haven't mentioned your OS type, I have to assume that it's Ubuntu, as you've placed this question in askubuntu.com.
Usually, the style configuration files will be under a folder named
resourcesorconfigurationthat is present inside the software installation path. For example,/opt/steam/resources/styleor/usr/share/steam/resources/styleor/opt/steam/config.The sub-folder name could be
styleorskins- depending on the browser/application.Now, check for a file that has the font (or, skin) configuration in it and change the font-type in it, according to your taste (you need to have
font-familyname ready, which you can get from your installed fonts in the system).Be careful not to change any other setting under the config folders - unless you are sure of the impact. Also, it's always wise to make a backup of the file that you're editing before making the changes.
Note: The folder names given above are just a guideline of the pattern to look for & not the exact folders, themselves.
I found the file
steam.stylesat~/.steam/debiad-installation-resource/stylesand adapted it to how I wanted.Reference: Steam Notifications Panel Modifications