Bookdown css file getting overwritten when book is rendered

I have a custom css file, mystyles.css, which works fine with one bespoke class when I render as a gitbook

I would like to use the sepia theme which is accomplished via fontsettings in the _output.yml file
However, the TOC is virtually unreadable so I attempted to address that via CSS in mystyles.css

Based on inspecting the HTML I tried

.book.color-theme-1 .book-summary {
    background: #c7c1c1;

But on rendering this part of the file disappears
The same applies if I even do something like

p {
  color: red;

Where am I going wrong?