Change font family for article titles

Hi! I’m using the Gamepress theme which has a built-in font family but it does not support my local language – some characters are displayed incorrectly. I managed to change the font of slider content, article content, etc. by adding several css codes to my custom css plugin. However I’m stuck with the titles of articles – what would be the correct code to use?

Looking at the page source info and the theme’s css file I found the post titles are under “entry-header” h2 section. But nothing changes when I try to insert following codes into my custom csc plugin like

h2.entry-header { font-family: Arial; }
#entry-header { font-family: Arial; }

What would be the right script to get this sorted out?

thanks for all your advise