The Unity desktop in Ubuntu Linux has some great features, but it also has some irritations too. For a start, the transparent Dash can make it difficult to read the text under icons and to navigate the sections. It is a stupid idea. Set the Launcher to auto-hide and it can be really hard to get it back on the screen. Here's how to make Unity less of a pain and more of a pleasure.
Ubuntu developers, please get a marker pen and write on a piece of paper, write on a window - the ultimate in transparency - and see which is easier to read. Paper wins hands down. When you try to read something on a transparent surface, you can't help looking through it and whatever is behind gets in the way and makes it hard to read the text or see images. Transparency is a clever graphical trick that programmers can perform, but it does not make a computer easier to use. In fact, it makes it harder.
You can't make the Dash have a solid background. I've searched high and low on the internet and not found a way to do it. I don't think it is possible. However, I did find a partial solution that I can share with you. I'm using Ubuntu 12.04 by the way.
Open Ubuntu Software Centre and enter compiz into the search box in the top right corner. Select and Install CompizConfig Settings Manager. Find it in the Dash afterwards and run it. Go to the Desktop section and open Ubuntu Unity Plugin. On the Behaviour tab you can set Hide Launcher to Autohide. The Launcher (the taskbar/Dock-like panel on the left) will now slide away when it is not in use. Note that the key to access it is Super (Windows key), which is useful to know.
Select the Experimental tab and there are several useful settings you can customise. Panel Opacity is the transparency of the top bar containing the icons and menus. A value of 0.000 is transparent and 1.000 is opaque. Similarly, the Launcher opacity can be set from 0 to 1 too. However, unlike the top panel it never becomes completely invisible. It shows a light colour even when set to fully transparent. I prefer it to be solid with a value of 1.
Getting the launcher to appear when it is set to auto-hide requires you to bang the mouse pointer really hard against the left-hand edge of the screen. It's something I don't like. You can modify the behaviour though. Reduce the Launcher Reveal Pressure to 10 or even 5 and the Launcher pops out much more easily.
Notice that there isn't an opacity setting for the Dash. Instead, there is a Dash Blur setting and it is either None (completely see through and not very useful), Static Blur or Active Blur. Active Blur works as you expect and when the Dash is displayed whatever is underneath is blurred. I personally don't like this. I find the text hard to read and even the icons aren't that clear when on a fuzzy background.
The solution is the Static Blur setting. When you start up Ubuntu and first use the Dash, whatever is behind it is blurred. However, this blurred background is stored and used every time you call up the Dash, regardless of what is underneath. So what you do is select a background wallpaper that is rather a plain in the top left corner of the screen, or better still, select a plain colour for the desktop instead of a photo. (Right click the desktop, select Change Desktop Background, Select Colours and Gradients from the drop-down menu.)
With a plain desktop colour, preferably a darkish one so the white text of the Dash stands out, the first time you use the Dash it will store it. Then no matter how busy it gets on the screen, the Dash will always be displayed on this plain background. In fact, you can change the desktop to photo wallpaper if you want. Just don't shut down because on the next startup it will store the background the first time the Dash is used.