Incorrect display in Gmail

I am testing a new template which shows up correctly in Outlook. Yahoo Mail and most others.

Gmail, though, is proving a real issue.

I have white text on a dark gray background for some links at the top. In all clients - except Gmail - these correctly show as white. In Gmail they are black.

I also have article headlines which are in a blue colour. In all clients these headlines show correctly. In Gmail they are turned black and also the type size is altered.

I have tried and retried but I cannot see what is wrong. When I look at the "show original" all the CSS is in tact in Gmail. Nothing has been altered along the way. So why isn't it displaying properly? Any ideas?

There is also another problem. Text which has no margins set (or even a zero bottom margin) is appearing in Gmail with large margins, yet tests on other email clients show no gaps.

Here is the CSS code.

.article-title {
    font-size: 20px;
    color: #27699E;
    font-weight: bold;
    font-family: "Trebuchet MS", Arial, Helvetica, sans-serif;
    margin-left: 25px;
    margin-right: 25px;
    margin-bottom: 3px;

.header-content {
    font-size: 11px;
    font-family: "Trebuchet MS", Arial, Helvetica, sans-serif;
    color: #FFFFFF;
    font-weight: bold;

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