Use chronos-lesson CSS class.

biscuit/apps/chronos/templates/chronos/tt_lesson.html

-<div class="card"
+<div class="card chronos-lesson"
  style="
   {% if lesson_period.lesson.subject.colour_fg %}
    color: {{ lesson_period.lesson.subject.colour_fg }};

biscuit/apps/chronos/templates/chronos/tt_week.html

@@ -20,7 +20,7 @@
     {% for num, times in periods.items %}
      <div class="row">
       <div class="col-sm">
-       <div class="card" style="width: 18rem; {% if lesson.colour_fg %} color: {{ lesson.colour_fg }}; {% endif %} {% if lesson.colour_bg %} background-color: {{ lesson.colour_bg }}; {% endif %}">
+       <div class="card chronos-lesson">
         <div class="card-body">
          <div class="row">
           <div class="col-sm">