Welcome Inn Theme Help

This is a general question i have a welcome inn theme and i have been modfying it with child css. The them itself support main menu up to 9 pages but i have put 10 th because i need it but that is not the problem.

Problem is the size. The theme is not responding to different screen resolutions. For example if i modify it with the 10th page in the main menu for 1600×900 which i use it will be usseles when i change it to 1024×768 and vice versa. So i hit a wall here and i need the help. How can i make it work for any resolution.