What you see may not be what others see
Lately, I have been evaluating some websites for others, and too often I have come across some major flaws in the website design of these sites.
These flaws are all connected to how the site shows on other screen sizes than your own.
Today most themes are responsive. That means that they should render and adapt your site perfectly on every screen. Sadly, that is far from the truth.
Yesterday I came across a site that most likely looked good on the 1024 screen width on the computer that it was constructed on, but on 1920 resolution, the header image was way too small and adjusted to the left and filling half the screen, while the text was centered.
I have seen pages that have built a post layout with images that looked good on 1280+ resolution but made no sense on mobile and tablet view.
Checking out your how website design looks on various screens should be a part of the QA done on every website in the constructing process.
Trusting that the theme, that claims to be responsive, handles this is not enough. You need to actually check it out.
Luckily there is help available to perform such checks.
To check a site on various screen resolutions and mobile views, I use: http://responsivedesignchecker...
If you want only mobile view you can use: http://www.responsinator.com
One thing you should look for in addition to the general appearance of your theme is how your menu works on mobile devices. Some themes render the menu in ways that make it appear very little user-friendly to your visitors.
Never trust that a responsive theme actually presents your website in the best possible way.
For further reading, you can check out this blog post of mine from earlier this year:
How I choose themes for my websites...