One of my lecturers liked to point out that if you ask people what you do, you seldom get a meaningful answer. I am hired as a graphic designer, which may make you think I spend 7,5h per day in Photoshop or Illustrator - the truth is slightly less glorious, I would guess roughly 2-3 hours a day are used for purely design-related activities. The rest is spent in meetings, coordination, bug reports and excessive coffee runs.
Ahem. Here are the simple steps to get started:
Before you start trying out your changes, mock them up in Photshop. That way it is easier to get feedback, and you can ensure that it will work from a design point of view before you get your hands dirty. Once it is good enough you should test it live, to make sure there are no technical limitations preventing your design from working.
Grab a fresh Opera build and install or unpack it. Unzip (to a location of you choosing) the standard_skin.zip for Windows & Linux changes or the mac_skin.zip for OS X related changes (note, I do not recommend this for creating a custom skin,
read this to find out why). The most interesting file inside these archives are the skin.ini file, as it controls the skinning. It can be read by any text editor.
In order to easier locate the element you want to change, you can enable a
magical setting - opera:config#UserPrefs|DebugSkin. It will display a giant yellow tooltip when you hover a skin element, giving you details about it, but perhaps especially important: the name of the section where you can find it in the skin.ini file.
To see the changes you go, point Opera to the extracted skin. Due to legacy reasons this is named "button set" - opera:config#UserPrefs|ButtonSet. All that is missing for the changes to appear is to reload the skin. The best way to do that is to go to Preferences > Advanced > Shortcuts > Keyboard Shotcuts. Once there, bind a key combination to "Reload Skin".
Then you are good to go! Type your text in the editor, save it, press key combo in Opera, done.