This seems to cover overdue activities only. However, as pointed out in #242, the fix needs to address more selectors (from CiviCRM core CSS):

.crm-container del,
.crm-container .crm-is_deleted,
.crm-container table.caseSelector td.status-urgent,
.crm-container .font-red,
.crm-container .status-removed,
.crm-container .status-overdue,
.crm-container .status-fatal,
.crm-container .status-hold,
.crm-container .status-past,
.crm-contact-deceased,
.crm-container .status-warning {
    color: #E43D2B !important;
}

Also, the fix makes the color rule apply to table cells within a .contact-activity-selector-activity element only, which might be ok, if that's the only place where these classes are used, but the original core styles indicate that these classes may be used anywhere within a .crm-container element.

@jensschuppe you are right, I should have said explicitly that this PR fixes only the specific issue that you reported in #242

Researching and troubleshoot all the potential locations that might be affected by seven and/or shoreditch will take much longer, and I wanted to have a quick solution in place in the meantime so that the usability of the Activities tab is not negatively affected by the theme

Sorry for the miscommunication!

No need to apologize, and thanks for working on it.

Maybe I didn't point out clearly enough that it's more than just overdue activities (as can be seen in the CSS I posted), and I chose those as an example to illustrate the issue.

For the issue of fixing the other selectors not getting lost, would you open #242 again for more commits or should we open another issue?

sure @jensschuppe , just did