Issues with bold text and its color

On my page I have a blog, lets take for example one blog post:

I used this code to ensure that the bold text in a blog post is shown in white (when I don’t use this code it appears as black):

.html_modern-blog .main_color strong {
color: #ffffff!important;
}

The problem now is that on the sidebar there is also bold text but is also shown in white right now, so that you can’t read the title of the shown 5 newest blog posts.

And also on the startpage I want to use bold text in the header section, but it will appear in white. How can I make sure that bold text only in single blog posts will be shown as white, so that the sidebar appears normal and I also can use bold text in the header section of the homepage without appearing white.

Do you know what I mean?