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span style=border-style:outset;border-color:#ffff00;border-width:3px;
This is code that would go in your layout code, not your css file. You should remove this completely.

If you want to change the font colour and size, you just have to change the following:

.top-menu li a {
    border-left: 1px solid #CCCCCC;
    color: blue;
    font-size: 16px;
    padding: 0 4px 0 8px;
}

See, my changes above would make the text blue and 16px large. That’s not necessarily the result you want, I know. But you can change the values to whatever you want, you just have to alter the color and font-size values individually.

I’m not sure what you mean by ‘colored outset’ though. Could you describe a little more the effect you’re going for?

While it’s out, maybe you could try deactivating the plugins to see if it has an immediate effect?

Could you try adding the image IN the sidebar? That would save you from editing the theme code.

1. Upload your image to your media library. Copy the url of the image location.
2. Go to Appearance > Widgets
3. Find the ‘Details’ sidebar
4. Add a text widget to the ‘Details’ sidebar, above any other widgets in the sidebar.
5. Add the code for the image location as html in your text widget.

Now, this might be affected by some styling issues particular to your theme, but it sounds like you can probably work those out yourself..

Has anything else changed recently? Have you updated your WordPress install, or added any new plugins?

The first step in troubleshooting would probably be to deactivate all your plugins and see if the problem still occurs. If that fixes it, then reactivate the plugins one at a time to determine which one is the bad guy.

What version of WordPress are you using?

This is it! You can change any of the values in this set of attributes in the style.css file of your child theme to make your changes.

.top-menu li a {
    border-left: 1px solid #CCCCCC;
    color: #333333;
    font-size: 11px;
    padding: 0 4px 0 8px;
}

Have you tried updating by uploading the wp files right to your ftp server, removing the need to go through the admin screen?

If you move ahead with that, make sure you don’t overwrite your /wp-content/ folder!!! And back it up first so you have a copy just in case.

Without a link to your live page, I can’t tell you the actual code for sure. Probably it would work if you reduced the top padding in this selector in your css (but you should create a child theme if you’re going to make changes):

#site-title {
    margin-right: 270px;
    padding: 3.65625em 0 0;
}

To be certain which code needs to be changed, you could use Firebug in Firefox to check out which css styles can be manipulated to reduce the height of that box, then create a child theme and add the change you need.

Not sure about the route you’ve taken, but thought I’d ask: have you checked out the Multilingual WordPress overview of options?