Sticky Menu Error on Post page


While I am uploading posts on our website I have found out that problem that the sticky/static menu had changed suddenly like as below. I think yesterday was ok, but now I am so embarrassed. Though I changed something seemed to be related on customized page but nothing changed. What should I do for inline (left to right) menu as original?

page address:

I’m not sure what cause it, but these css code can help you to fix the issue, you can use custom css plugin to add the code below:

#mainnav ul li {
    width: auto;

where should I copy it?

you can use “custom css plugin”, download and install it.