Thunderbird addon
I just updated the extension QuoteColors; it is released under the name New QuoteColors, note that the extension's xpi file is signed to ease the installation. The extension does not handle the message composer/editor, this require additional work to make it compatible with that section, i will edit my answer later on if i have the time to work on this.
Sources & site: Github page
Download: v0.3.5 release (compatible v60.x and v68.x)
Mozilla addons site: addons.thunderbird.net (still under review)
Custom CSS:
Customizing the message composer while in plain-text mode is not an easy task as plain-text editing does not have any css and thus can not handle css customisations, BUT this depend on the way you are applying plain-text because Thunderbird can be setup to compose the message in html while sending it in plain-text.
please test the following code and let me know ;) note that this is an exhaustive custom CSS list for the matter from TB v68.4.1 for testing purpose while technical details about your setup are missing
span[_moz_quote=true] {
color: green !important;
background: green !important;
background-color: green !important;
border-color: green !important;
}
pre[_moz_quote=true] {
color: green !important;
background: green !important;
background-color: green !important;
border-color: green !important;
}
/* :::: ---------------------------------------------------------------------------------------- ::::: */
/* ::::: message text, incl. quotes ::::: */
.moz-text-flowed blockquote,
.moz-text-plain blockquote {
border-color: red !important;
color: red !important;
background: red !important;
background-color: red !important;
}
.moz-text-plain pre {
border-color: red !important;
color: red !important;
background: red !important;
background-color: red !important;
}
.moz-text-plain[wrap="true"] {
color: red !important;
background: red !important;
background-color: red !important;
border-color: red !important;
}
.moz-text-plain[wrap="false"] {
color: red !important;
background: red !important;
background-color: red !important;
border-color: red !important;
}
.moz-text-plain[wrap="flow"] .moz-txt-sig {
color: red !important;
background: red !important;
background-color: red !important;
border-color: red !important;
}
.moz-text-plain[graphical-quote="false"] blockquote {
color: red !important;
background: red !important;
background-color: red !important;
border-color: red !important;
}
.moz-text-plain[graphical-quote="true"] .moz-txt-citetags {
color: red !important;
background: red !important;
background-color: red !important;
border-color: red !important;
}
.moz-txt-underscore {
color: red !important;
background: red !important;
background-color: red !important;
border-color: red !important;
}
.moz-txt-formfeed {
color: red !important;
background: red !important;
background-color: red !important;
border-color: red !important;
}
/* :::: ---------------------------------------------------------------------------------------- ::::: */
body.light blockquote {
border-inline-start: 2px solid blue !important;
color: red !important;
background: red !important;
background-color: red !important;
border-color: red !important;
}
body.sepia blockquote {
border-inline-start: 2px solid blue !important;
color: red !important;
background: red !important;
background-color: red !important;
border-color: red !important;
}
body.dark blockquote {
border-inline-start: 2px solid blue !important;
color: red !important;
background: red !important;
background-color: red !important;
border-color: red !important;
}
.moz-reader-content blockquote {
color: red !important;
background: red !important;
background-color: red !important;
border-color: red !important;
}
/* :::: ---------------------------------------------------------------------------------------- ::::: */
body blockquote[type=cite] {
color: red !important;
background: red !important;
background-color: red !important;
border-color: red !important;
}
/* :::: ---------------------------------------------------------------------------------------- ::::: */
body blockquote[type=cite] {
color: red !important;
background: red !important;
background-color: red !important;
border-color: red !important;
}
blockquote[type=cite] blockquote[type=cite] {
color: red !important;
background: red !important;
background-color: red !important;
}
blockquote[type=cite] blockquote[type=cite] blockquote[type=cite] {
color: red !important;
background: red !important;
background-color: red !important;
border-color: red !important;
}
blockquote[type=cite] blockquote[type=cite] blockquote[type=cite] blockquote[type=cite] {
color: red !important;
background: red !important;
background-color: red !important;
}
blockquote[type=cite] blockquote[type=cite] blockquote[type=cite] blockquote[type=cite] blockquote[type=cite] {
color: red !important;
background: red !important;
background-color: red !important;
}
/* :::: ---------------------------------------------------------------------------------------- ::::: */
.moz-email-headers-table,
.moz-email-headers-table > tbody > tr > th,
.moz-email-headers-table > tbody > tr > td,
blockquote[type=cite] table,
blockquote[type=cite] table > tbody > tr > th,
blockquote[type=cite] table > tbody > tr > td {
border: 1px solid transparent !important;
color: red !important;
background: red !important;
background-color: red !important;
border-color: red !important;
}
table.moz-email-headers-table:hover,
table.moz-email-headers-table:hover > tbody > tr > th,
table.moz-email-headers-table:hover > tbody > tr > td,
blockquote[type=cite] table:hover,
blockquote[type=cite] table:hover > tbody > tr > th,
blockquote[type=cite] table:hover > tbody > tr > td {
color: red !important;
background: red !important;
background-color: red !important;
border-color: red !important;
}
/* :::: ---------------------------------------------------------------------------------------- ::::: */
body > p:-moz-first-node, td > p:-moz-first-node, th > p:-moz-first-node,
body > dl:-moz-first-node, td > dl:-moz-first-node, th > dl:-moz-first-node,
body > multicol:-moz-first-node, td > multicol:-moz-first-node, th > multicol:-moz-first-node,
body > blockquote:-moz-first-node, td > blockquote:-moz-first-node, th > blockquote:-moz-first-node,
body > h1:-moz-first-node, td > h1:-moz-first-node, th > h1:-moz-first-node,
body > h2:-moz-first-node, td > h2:-moz-first-node, th > h2:-moz-first-node,
body > h3:-moz-first-node, td > h3:-moz-first-node, th > h3:-moz-first-node,
body > h4:-moz-first-node, td > h4:-moz-first-node, th > h4:-moz-first-node,
body > h5:-moz-first-node, td > h5:-moz-first-node, th > h5:-moz-first-node,
body > h6:-moz-first-node, td > h6:-moz-first-node, th > h6:-moz-first-node,
body > listing:-moz-first-node, td > listing:-moz-first-node, th > listing:-moz-first-node,
body > plaintext:-moz-first-node, td > plaintext:-moz-first-node, th > plaintext:-moz-first-node,
body > xmp:-moz-first-node, td > xmp:-moz-first-node, th > xmp:-moz-first-node,
body > pre:-moz-first-node, td > pre:-moz-first-node, th > pre:-moz-first-node,
body > ul:-moz-first-node, td > ul:-moz-first-node, th > ul:-moz-first-node,
body > menu:-moz-first-node, td > menu:-moz-first-node, th > menu:-moz-first-node,
body > dir:-moz-first-node, td > dir:-moz-first-node, th > dir:-moz-first-node,
body > ol:-moz-first-node, td > ol:-moz-first-node, th > ol:-moz-first-node {
color: red !important;
background: red !important;
background-color: red !important;
border-color: red !important;
}
userContent.css
that wont be easy because of the plaintext format (no css) – intika Jan 20 '20 at 13:44