Fix the dark theme text color

Signed-off-by: GretaD <gretadoci@gmail.com>

src/components/MessageHTMLBody.vue

 <template>
 	<div id="mail-content">
 		<MdnRequest :message="message" />
-		<div v-if="hasBlockedContent" id="mail-message-has-blocked-content">
+		<div v-if="hasBlockedContent" id="mail-message-has-blocked-content" style="color: #000000">
 			{{ t('mail', 'The images have been blocked to protect your privacy.') }}
 			<button @click="onShowBlockedContent">
 				{{ t('mail', 'Show images from this sender') }}
@@ -67,7 +67,7 @@ export default {
 				// send css vars to client page
 				this.$refs.iframe.iFrameResizer.sendMessage({
 					cssVars: {
-						...getCssVar('--color-main-text'),
+						...getCssVar('--color-main-background'),
 					},
 				})
 			},
@@ -127,6 +127,7 @@ export default {
 	display: flex;
 	flex-direction: column;
 	height: 100%;
+	background-color: #FFFFFF;
 }
 #mail-message-has-blocked-content {
 	margin-left: 8px;