Tiny Mci & interfering CSS code 5.6.3

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Within a Stack I have a HTML block containing a table with associated css within the block. The line:
tr td:hover { background: #c7005a; color: #FFF; }
that colours the cells on mouseover is causing that same background colour to appear when I use Tiny Mci anywhere else within the site. It appears behind all the office icons when using the text processor when inserting a content block. See little red background in attachment!

Is this an issue of having the CSS style all within the same block as the html?

thanks

Robert

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