Oh, this highlight-color would be quite useful too for a macro-variation I'm currently researching for like crazy, for temporary stylesheets in new toolbar... So I just looked this up too
Must admit am rather clueless, and so far no luck for the automatic highlighting (hightlight-button), but at least for "Find next" found two solutions:
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SOURCE:
http://www.dslreports.com/forum/remark,13892255
Add this to your user stylesheet:
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/*Change color of highlighted text*/
::-moz-selection {
color: Black !important; /*Colour of text*/
background: fuchsia !important; /*Highlighting colour*/}
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There is a SECOND solution, works instantly without KM restart, and so is really cool!
SOURCE:
http://www.neowin.net/forum/lofiversion/index.php/t284675.html
Type "about:config", and add the option "ui.textSelectBackground" (type string, value some color name like fuchsia)
Only problem, doesn't seem to work if there's a customized background in a (temporary) user stylesheet, looks like its settings have priority...
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By the way, aside from those cryptic hex-colors there are also some 140 "real" color names that can be used in a CSS stylesheet, for backgrounds or fonts or anything colored. One of the many lists is here
http://www.mechatronika.hu/~mikdor/kieg/web/colors.html