I can replicate the issue using Web Outlook App on Microsoft Edge (Version 106.0.1370.47 (Official build) (64-bit)), Firefox 106.0.1 (64-bit) and Chrome (Version 106.0.5249.119 (Official Build) (64-bit)) on Windows 10 Enterprise, version 20H2.
Windows 10 Settings > Personalisation > Colours >
Then either:
Choose your colour: Custom.
Choose your default app mode: Dark.
or:
Choose your colour: Dark.
Outlook Web App > Settings > Dark mode: Off.
Seems to be the color properties in this CSS style taken from a new version email (as an example) below?:
<div class="rps_735d"><div dir="ltr" style="line-height:1.4; text-align:left"><p>Hooray, a new version of <a href="https://www.discourse.org" target="_blank" rel="noopener nofollow ugc noreferrer" data-auth="NotApplicable" style="text-decoration:none; font-weight:bold; color:#006699" data-safelink="true" data-linkindex="0"><span data-markjs="true" class="mark95gsb08l7" style="background-color: rgb(255, 241, 0); color: black;" data-ogac="" data-ogab="" data-ogsc="" data-ogsb="">Discourse</span></a> is available!</p><p>Your version: 2.9.0.beta9<br aria-hidden="true">New version: <strong>2.9.0.beta10</strong></p><ul style="margin:0 0 0 10px; padding:0 0 0 20px"><li style="padding-bottom:10px"><p>Upgrade using our easy <strong><a href="https://forum.step.org/admin/upgrade" target="_blank" rel="noopener noreferrer" data-auth="NotApplicable" style="text-decoration:none; font-weight:bold; color:#006699" data-safelink="true" data-linkindex="1">one-click browser upgrade</a></strong></p></li><li style="padding-bottom:10px"><p>See what’s new in the <a href="https://meta.discourse.org/tag/release-notes" target="_blank" rel="noopener nofollow ugc noreferrer" data-auth="NotApplicable" style="text-decoration:none; font-weight:bold; color:#006699" data-safelink="true" data-linkindex="2">release notes</a> or view the <a href="https://github.com/discourse/discourse/commits/main" target="_blank" rel="noopener nofollow ugc noreferrer" data-auth="NotApplicable" style="text-decoration:none; font-weight:bold; color:#006699" data-safelink="true" data-linkindex="3">raw GitHub changelog</a></p></li><li style="padding-bottom:10px"><p>Visit <a href="https://meta.discourse.org" target="_blank" rel="noopener nofollow ugc noreferrer" data-auth="NotApplicable" style="text-decoration:none; font-weight:bold; color:#006699" data-safelink="true" data-linkindex="4">meta.<span data-markjs="true" class="mark95gsb08l7" style="background-color: rgb(255, 241, 0); color: black;" data-ogac="" data-ogab="" data-ogsc="" data-ogsb="">discourse</span>.org</a> for news, discussion, and support for <span data-markjs="true" class="mark95gsb08l7" style="background-color: rgb(255, 241, 0); color: black;" data-ogac="" data-ogab="" data-ogsc="" data-ogsb="">Discourse</span></p></li></ul><h3 style="margin:30px 0 10px"><a href="#x_release-notes-1" name="x_release-notes-1" style="text-decoration:none; font-weight:bold; color:#006699" data-safelink="true" data-linkindex="5"></a>Release notes</h3><p>Read about the release here:</p><p><a href="https://meta.discourse.org/t/2-9-0-beta10-sidebar-new-notification-menu-security-fixes-and-more/239858" target="_blank" rel="noopener nofollow ugc noreferrer" data-auth="NotApplicable" style="text-decoration:none; font-weight:bold; color:#006699" data-safelink="true" data-linkindex="6">https://meta.<span data-markjs="true" class="mark95gsb08l7" style="background-color: rgb(255, 241, 0); color: black;" data-ogac="" data-ogab="" data-ogsc="" data-ogsb="">discourse</span>.org/t/2-9-0-beta10-sidebar-new-notification-menu-security-fixes-and-more/239858</a></p><p>This release includes security fixes, so please update as soon as possible.</p><div style="display:none; white-space:nowrap; font:15px courier; line-height:0"> </div><style>
<!--
@media (prefers-color-scheme: dark) {
.rps_735d html
{background:#151515!important}
.rps_735d h1, .rps_735d h2, .rps_735d h3, .rps_735d h4, .rps_735d h5, .rps_735d h6, .rps_735d p, .rps_735d span, .rps_735d td
{color:#dddddd!important}
.rps_735d [data-stripped-secure-media]
{border-color:#454545!important}
.rps_735d [dm='text-color']
{color:#dddddd}
.rps_735d [dm='header']
{background:#151515!important}
.rps_735d [dm='topic-body']
{background:#151515!important;
border-bottom:1px solid #454545!important}
.rps_735d [dm='triangle']
{border-top-color:#151515!important}
.rps_735d [dm='body']
{background:#222222!important;
color:#dddddd!important}
.rps_735d [dm='body_primary']
{background:#062e3d!important;
color:#dddddd!important}
.rps_735d [dm='bg']
{background:#323232!important;
border-color:#454545!important}
}
-->
</style></div></div>
A workaround in the meantime would be to suggest to the user that they could turn dark mode on in Outlook Web App Settings…?