Open the Opera web browser. Click the "Menu" button in the upper left corner of the browser window.
Select "Appearance." Alternatively, you can use the keyboard shortcut "Shift+F12" to open the appearance menu.
Scroll down to the "Color Scheme" section. Click on the arrow to display the available color schemes.
Select the color scheme you want to use. For example, if you want to use red, select "Red." Click "Ok." The browser's panel is updated automatically.