Soap Themes

Soap themes are a convention in their own right in that each soap has a memorable theme tune, with a defining feature, our two examples being the drum roll in EastEnders, and the Guitar riff from Hollyoaks. This is usually to draw the attention of a viewer for quick and easy recognition of the show.
An explanation relating to this phenomenon is described by Louis Althusser in his theory of interpellation, which refers to church bells in the old days when people may not have access to the time, and so the bells would signal the beginning of church. Likewise, the beginning of the theme is usually the catchiest and signals the beginning of the show.
EastEnders
For EastEnders, the drum roll and theme tune had remained for years, and is so well known to the viewers that when they changed it, there was a large amount of complaints, and had to be changed back due to popular demand. The intro also remained the same, changing only to fit with the contemporary setting of the Thames and architectural additions:

(With correct timing it's eerie comparing the similarities between old and new...)

The tune also has a soundbridge at the end making the cut into the episode seem more fluent and natural.

The theme also is continuous, slow paced and fits perfectly with the visuals, in addition, the instruments used are classic, with pianos, drums and even clapping.

Hollyoaks:

Hollyoaks, being geared towards the youth culture demographic is a drastically different theme to EastEnders:

Due to the soap being geared toward a younger generation to EastEnders, the intro is performed by more contemporary instruments, such as guitars and a synthesiser. The imagery is also much more vivid, with multiple cuts to each character with colourful, computer animated backgrounds reflecting their personalities.