Summarize the following:
Go to https://www.twitter.com/ in your computer's web browser. This will open your Twitter home page if you're logged in. If you aren't logged in, enter your Twitter email address (or username) and password before proceeding. It's the circular icon in the top-right corner of the page. A drop-down menu will appear. This option is in the drop-down menu. Doing so takes you to your Twitter profile page. It's below the bottom-right corner of your profile page's cover photo. You'll find this option on the left side of the profile page. Doing so opens a section with several different-colored boxes. It's in the lower-right side of the section with colored boxes. This will open a text field. If you just want to use a preset color, instead click the color you want to use and then skip the next step. Type your color code into the text box. You should see the box with the ＋ in it change color to reflect your chosen hue. It's in the upper-right side of the page. Doing so will apply the theme to your Twitter profile.

Summary:
Open Twitter. Click your profile icon. Click Profile. Click Edit profile. Scroll down and click Theme color. Click ＋. Enter your color's code. Scroll up and click Save changes.